Dec. 15, 2010 -- Doctors who came into contact with William Hamman, the pilot who passed himself off as a cardiologist, had no idea he wasn't who he said he was. They thought he was an accomplished cardiologist who also flew.

"He was quite good at what he did (which was never patient care)," Dr. Cindy Grines, an interventional cardiologist at William Beaumont Hospital in Royal Oak, Mich., wrote in an e-mail. She worked with him at the hospital and was impressed by knowledge.

"He focused on his experience as a pilot, and used these skills to develop computer simulations, to implement team strategies to expedite patient care and to develop check lists to assure ACC [American College of Cardiology] recommendations were followed," Grines added. She also praised his ability to perform complicated procedures as well as teach other doctors.

Hamman is a licensed pilot who worked for United Airlines until the airline grounded him when his lies came to light. In addition to the real credentials he possessed, he also had ones he made up, including M.D. and Ph.D. degrees, as well as 15 years of clinical experience as a cardiologist. In reality, he did attend medical school for a few years, but never graduated.

"I remember he gave grand rounds [presentations of patients' medical conditions and treatments] at Beaumont," said Dr. Peter A. McCullough, now a consultant cardiologist at St. John Providence Health System in Novi, Mich. "He was very confident and smooth in his presentation."

Doctors at other medical centers were also impressed by lectures he gave at meetings and conferences. Hamman gave a presentation at the American College of Cardiology's annual meeting this past March.

"He seemed to be a bright guy and wanted to teach others how to perform better in the team environment," said Dr. W. Douglas Weaver, head of the Division of Cardiovascular Medicine at Henry Ford Hospital in Detroit. Weaver is a past president of the American College of Cardiology.

Weaver said Hamman only talked about how doctors can work better as a team and not about how to practice medicine. Regardless, Weaver said he was still surprised to hear about Hamman's deception.

"I was shocked to learn he was not a cardiologist, as that was how he advertised himself, although said he had never practiced. He made a big mistake, and the funny part about it was he did not have to be a physician to be effective in his role as an educator," Weaver said.

Better Credential Checks Needed, Say Doctors

Cardiologists say that imposters like Hamman show the need for national medical organizations to perform stricter checks on physicians, making sure they have the degrees and licenses they say they have.

"We need to be vigilant that the credentials of physicians are real, current and that the doctor is board certified and participating in their board's lifelong learning program," said Dr. Richard J. Shemin, chief of the Division of Cardiothoracic Surgery at the Ronald Reagan UCLA Medical Center in Los Angeles.

Hamman gave several lectures at continuing medical education (CME) conferences, designed to provide physicians with knowledge they need to retain and sharpen their skills in their specialties.

"Usually CME course directors have some direct knowledge of the body of work done by experts that qualifies them to lecture," said Dr. William O'Neill, who was a cardiologist at Beaumont before becoming executive dean for clinical affairs at the University of Miami School of Medicine. "CME courses themselves do not require background checks or verification of credentials. The AMA [American Medical Association] and ACC were victimized. Somehow this man was able to pass himself off as an expert."

The ACC said it has since severed its relationship with Hamman. The information he presented at this year's annual meeting was peer reviewed and was appropriately accredited, so they say they stand by the material he presented at the meeting.

A Beaumont spokeswoman told the Associated Press Hamman resigned after the hospital checked his credentials and discovered he was an imposter.

While Hamman may be an embarrassment to some of the medical community, others say they are relieved he never did any harm to patients.

"There is no way this guy could have used this ruse to take care of patients (it would have triggered far more serious credentialling)," Dr. Cam Patterson, chief of cardiology at the University of North Carolina Chapel Hill School of Medicine in Chapel Hill, N.C., wrote in an e-mail.

"Fortunately, unlike some other imposters, it appears that he did not put patients directly at risk," said Dr. Alan Kadish, formerly a professor of medicine at Northwestern University's Feinberg School of Medicine.

ABC News' Sharyn Alfonsi and Lee Ferran contributed to this report.