Conventional wisdom holds that a college education is the key to success. But for entrepreneurs who dropped out to start their own companies, what is the point of dropping back in, especially if you're already running a successful business? After all, college dropouts Bill Gates (Harvard), Steve Jobs (Reed College), and Michael Dell (University of Texas) did just fine without a bachelor's degree.

"Being an entrepreneur is part rational and part emotional," says Andrew Zacharakis, professor of entrepreneurship at Babson College in Wellesley, Mass. "Successful entrepreneurs are passionate about what they do, and sometimes this means leaving sooner to get on with it."

For Michael Brown, "it" was starting mortgage brokerage firm Wells Street Financial in Chicago. Just two semesters shy of a bachelor's degree, Brown dropped out of the University of Michigan to run his business, which employs three people and handles about $60 million worth of financing annually. His decision to return to college was based on both professional and personal reasons. First, he wanted the credibility that a college degree confers. "In the long term, I want to raise money from pension funds," says Brown, 25. "I have a fiduciary responsibility, and I think it would be tough for them to see that in someone without a college degree." And then there was the promise he made to his mother to graduate, something his father never did.

Deciding to return is easier said than done. For a year, Brown took three night courses in Chicago while running his business during the day. Eventually, he moved back to his parents' house in suburban Detroit and returned to his alma mater full-time to major in history with an emphasis in business. "I was a dinosaur in finance class, but the kids looked up to me," he says. "I knew tons of stuff based on real-world experience. We got A's on all of our group projects." Outside the classroom, Brown focused on existing accounts instead of courting new ones, which cost him lucrative opportunities. He managed by delegating part of the workload to his one employee at the time and relying on technology -- checking in with clients between classes or via BlackBerry during a 45-minute commute to campus.

"Successful entrepreneurs are passionate. Sometimes this means leaving sooner to get on with it."

Whereas Brown finally got his degree within four months, Alex Fortunati, CEO of outsourcing firm Support Services America in Norwalk, Calif., expects to frame his in five years. The 45-year-old dropped out of Catholic University in Buenos Aires during his freshman year because, true to the entrepreneurial mindset, he thought he could learn business on his own. He hasn't done a bad job: The 120-employee SSA earned $10 million in revenue and made the Inc. 500 at No. 143 last year. Still, "I want a larger business," he says. "I want to show reliability and that I have an education to creditors, bankers, and potentially large customers. I realized that I needed to upgrade my education."

He's now enrolled at Excelsior College in Albany, N.Y., which allows him to take courses and seminars at local accredited colleges and apply them toward his B.A. in liberal arts. Typically, Fortunati works 12 hours a day but ducks out early twice a week for an evening class and is often finishing homework at 1 a.m. "It's tough. I only have a few days of quality time for my family," says Fortunati, the father of four. "I want my children to go to college. This is the right way to put it across to them that this is important."

If anyone can keep up with the demands of a grueling schedule, it's an entrepreneur, accustomed to doing everything himself. Not only does Alasdair McLean-Foreman balance college and HDO Sport, a high-tech sporting goods company he founded in his freshman dorm, he's also on Harvard's track team and training for the Olympic trials for his native England. Now 22, McLean-Foreman dropped out his sophomore year to build HDO's online business, as well as a retail outlet in Boston.

"The company would be more productive if I hadn't gone back," he says. "But there is plenty of time in the future. There is no ceiling to what we can do." He, like Brown, shifted day-to-day tasks to the staff of five; he wedges marketing and finance duties between classes and track meets, laptop and BlackBerry in tow. "I don't," he admits, "have the regular college experience."

Other dropouts never have the experience at all. For them, the world becomes their classroom and real life trumps the diploma. "Being an entrepreneur takes more than a degree," says Steve Nickerson, 34, CEO of real estate marketing company HomeRoute in Howell, Mich. "It takes ideas and the ability to execute them. I didn't get that in college." So Nickerson left the University of Michigan during his junior year to start his company, which earned $2 million in revenue and made the Inc. 500 at No. 332 last year. Although he was studying toward a degree in business administration, Nickerson says that "being in the business world taught me about business. College is not a prerequisite if you are going to create a company."

In the end, going back for a bachelor's degree even after professional success comes from a deeply personal place. In 2002, an entrepreneur with two companies -- and honorary doctorates from Yale and Brown -- graduated from California State University Long Beach at the age of 55, 33 years after he dropped out. At the time he said, "I wanted to accomplish this as a thank-you to my parents for giving me the opportunity for an education and a career." His name? Steven Spielberg.