I’VE BEEN SEEING this guy since I was a little kid,” said Cameron Ryan in amazement, sitting on the tan leatherette examining table. “Since I was 18 months old!”

He’s now a very big kid: six-five and close to 300 pounds, a high school senior with a smooth precocious basso and a passing resemblance to Magic Johnson. When Ryan first sat on this examining table, “this guy,” Dr. Martin Cahn, was a young MD struggling to establish his practice. He had a bushy beard and a halo of reddish hair and he wore a tie because doctors were supposed to. Today ties are a distant memory, and Cahn wears sneakers at the office. His hair and beard are salt-and-cinnamon. He has a middle-aged spread and an expression that flits between solicitude, exasperation, and, at the moment, glowing pride.

This visit, a routine checkup, was also a personal milestone for Ryan, and an example of the difference an old-fashioned family doctor can make in one patient’s life, above and beyond what’s commonly labeled “medicine.” Ryan is as ebullient as the toddler he was when he first saw Cahn. “I’ve lost weight,” he said proudly. “I want to get down to 280. But I assume my blood pressure is high….” Cahn checked it, frowned darkly, but said nothing; he didn’t want to kill his patient’s buzz.

Ryan had big news. Next year he’ll be attending a top-flight school, Purdue University. “And it was this guy who got me to do it,” he beamed, pointing to his doctor. “He was the one who told me, ‘Apply to colleges!’ even when I didn’t think I could do it.”

A year ago it was by no means obvious Ryan could do it. “You’ve always been an extraordinarily smart guy, Cameron,” Cahn told him. “You just needed to focus.” In junior year Ryan was flailing. Then a belated diagnosis of attention deficit hyperactivity disorder and the resultant drug regimen turned him around. His grades and SAT scores soared.

“Any idea what you’re going to major in?” Cahn asked.

“I don’t know, probably psychology. I’m also interested in business, but I don’t know if there will be any jobs in it when I come out. I figure there will always be work helping messed-up people.”

“My advice is to have a great time at college and try as many things as you can,” said Cahn, and for the second time that day he segued into The Story, the one he likes to tell patients trying to figure out how to live their lives…

Martin Cahn grew up in Chicago, Lexington (Massachusetts), and Schenectady. His father, John Cahn, is a celebrated metallurgist; he received the National Medal of Science and was bruited as a Nobel candidate. Young Marty set out in his father’s traces. He attended MIT, where his father once taught, and majored in biology.

Next: What made Cahn choose primary care, and how he remembers every patient in his 7,356 database.