The Internet can deliver smoking cessation interventions to large numbers of smokers. Little is known about the feasibility, reach, or efficacy of Internet cessation interventions. Virtually no data exist on who enrolls in cessation programs or on differences between those who complete enrollment and those who do not. This paper reports recruitment and enrollment findings for the first 764 participants in an ongoing randomized controlled trial that tested the efficacy of a widely disseminated Internet smoking cessation service (www.QuitNet.com) alone and in conjunction with telephone counseling. Study participants were recruited through Internet search engines using an active user sampling protocol. During the first 16 weeks of the study, 28,297 individuals were invited. Of those, 11,147 accepted the invitation, 5,557 screened eligible, 3,614 were recruited, 1,489 provided online informed consent, and 764 were confirmed eligible and enrolled. Of those who were at least curious about a cessation trial (n= 11,147), 6.9% enrolled. Of those who were eligible and recruited (n=3,614), 21.1% enrolled. Depending on the denominator selected, results suggest that 7% to 21% of smokers interested in cessation will enroll into a research trial. Internet recruitment provides unique challenges and opportunities for managing sample recruitment, analyzing subsamples to determine generalizability, and understanding the characteristics of individuals who participate in online research.