Internet interventions for smoking cessation are ubiquitous. Yet, to date, there are few randomized clinical trials that gauge their efficacy. This study is a randomized clinical trial (N=284, n=140 in the treatment group, n=144 in the control group) of an Internet smoking cessation intervention. Smokers were randomly assigned to receive either bupropion plus counseling alone, or bupropion and counseling in addition to 12 weeks of access to the Comprehensive Health Enhancement Support System for Smoking Cessation and Relapse Prevention (CHESS SCRP; a Web site which provided information on smoking cessation as well as support). We found that access to CHESS SCRP was not significantly related to abstinence at the end of the treatment period (OR=1.13, 95% CI 0.66–2.62) or at 6 months postquit (OR=1.48, 95% CI 0.66–2.62). However, the number of times participants used CHESS SCRP per week was related to abstinence at both end of treatment (OR=1.79, 95% CI 1.25–2.56) and at the 6-month follow-up (OR=1.59, 95% CI 1.06–2.38). Participants with access to CHESS SCRP logged in an average of 33.64 times (SD=30.76) over the 90-day period of access. Rates of CHESS SCRP use did not differ by ethnicity, level of education or gender (all p>.05). In sum, results suggest that participants used CHESS SCRP frequently, CHESS SCRP use was related to success, but the effects in general did not yield intergroup effects.