Introduction: On-line continuing medical education (CME) provides advantages to physicians and to medical educators. Although practicing physicians increasingly use on-line CME to meet their educational needs, the overall use of on-line CME remains limited. There are few data to describe the physicians who use this new educational medium; yet, they clearly are the innovators and early adopters who will facilitate the growth of this educational technology. It would be useful to instructional designers and CME developers to better understand the characteristics of this influential group. Methods: We studied the actual use of several different on-line CME programs within three different groups of physicians. The on-line programs were developed as part of research studies funded by the National Institutes of Health, with no relationship to commercial interests. They were presented to physicians using mass mailouts (two physician groups) or personal contact and were accompanied by incentives to reduce resistance to the new technology. We compared the characteristics of physicians who chose to use these on-line programs with demographic data from larger populations representing the groups from which these users originated. Results: We found that physicians who used these on-line CME programs were younger than average and, importantly, more likely to be female than expected. This finding was consistent across different types of physician populations and different types of CME programs. Discussion: Based on data reflecting actual use of on-line CME, younger physicians appear to be adopting on-line CME more rapidly than others, and women physicians appear to be adopting on-line CME at a faster rate than their male counterparts. This latter finding conflicts with the impression provided by some survey-based studies that male physicians are more likely than female physicians to use on-line CME. The data suggest that the growth of on-line CME is most likely occurring in diffusion networks dominated by relatively new medical school graduates and, possibly, women physicians. These results provide valuable insight to those who seek to develop and market on-line CME and those who seek to reach women physicians with CME programs. (Contains 3 tables.)