UNTIL nine years ago, Dr. Neal Grossman didn’t make a habit of parading around his bedroom in his skivvies and admiring his physique in the mirror. Nor did he ever imagine that his oldest son, then a teenager, would take one look at his father midflex and cry out, “Dad, put your clothes back on!”

But now that Dr. Grossman, a 60-year-old Baltimore dentist, is a competitive amateur bodybuilder, an extra ounce of flab makes the difference between a sizable trophy and going home empty-handed. “The minute you start winning, that’s all the validation people need to accept what you do as legitimate and something to be appreciated,” said Dr. Grossman, who is 5-foot-2 and a chiseled 121 pounds.

He is one of a small but growing number of 60- and 70-year-old bodybuilders stripping down to Speedos, slathering on bronzer, and strutting their stuff onstage in natural, or drug-free, competitions. The season for amateur and pro-level events begins this month.

Last year, the World Natural Sports Organization, one of about a dozen bodybuilding groups devoted to drug-free contests, had 44 competitors older than 60, up from two in 2000, said Jeffrey Kippel, a founder.