Background: The availability of Internet-based continuing medical education is rapidly increasing, but little is known about recruitment of physicians to these interventions. Objective: The purpose of this study was to examine predictors of physician participation in an Internet intervention designed to increase screening of young women at risk for chlamydiosis. Methods: Eligibility was based on administrative claims data, and eligible physicians received recruitment letters via fax and/or courier. Recruited offices had at least one physician who agreed to participate in the study by providing an email address. After one physician from an office was recruited, intensive recruitment of that office ceased. Email messages reminded individual physicians to participate by logging on to the Internet site. Results: Of the eligible offices, 325 (33.2%) were recruited, from which 207 physicians (52.8%) participated. Recruited versus nonrecruited offices had more eligible patients (mean number of eligible patients per office: 44.1 vs 33.6; P <.001), more eligible physicians (mean number of eligible physicians per office: 6.2 vs 4.1; P <.001), and fewer doctors of osteopathy (mean percent of eligible physicians per office who were doctors of osteopathy: 20.5% vs 26.4%; P =.02). Multivariable analysis revealed that the odds of recruiting at least one physician from an office were greater if the office had more eligible patients and more eligible physicians. More participating versus nonparticipating physicians were female (mean percent of female recruited physicians: 39.1% vs 27.0%; P =.01); fewer participating physicians were doctors of osteopathy (mean percent of recruited physicians who were doctors of osteopathy: 15.5% vs 23.9%; P =.04) or international medical graduates (mean percent of recruited physicians who were international graduates: 12.3% vs 23.8%; P =.003). Multivariable analysis revealed that the odds of a physician participating were greater if the physician was older than 55 years (OR = 2.31; 95% CI = 1.09-4.93) and was from an office with a higher Chlamydia screening rate in the upper tertile (OR = 2.26; 95% CI = 1.23-4.16). Conclusions: Physician participation in an Internet continuing medical education intervention varied significantly by physician and office characteristics.