Introduction: Physician use of online continuing medical education (CME) is growing, but there are conflicting data on the uptake of online CME and few details on this market. Methods: Analyses of 11 years of data from the Accreditation Council for Continuing Medical Education (ACCME) and a survey of 272 publicly available CME Web sites. Results: The data suggest that online CME was 6.9%-8.8% of CME consumed in 2008. If previous exponential growth continues, online CME is likely to be 50% of all CME consumed within 7-10 years. Most (60%) online CME is produced by medical publishing and education companies. The online CME marketplace is consolidating, with 16% of surveyed sites providing 76% of available credits. Currently, 70% of online CME is offered at $10 or less per credit. Most online CME uses low-technology educational approaches, such as pure text and repurposed live lectures. Discussion: Online CME use is growing rapidly and is likely to be half of all CME consumed by practicing physicians within a few years. The pattern is consistent with Christensen's model of "disruptive innovation", whereby an innovative technology eventually displaces an incumbent technology by first providing a relatively low-quality, low-cost product that meets the needs of unserved customers. The technologies being developed for online CME may facilitate broader changes in medical education as well. (Contains 3 tables and 1 figure.)