Immunotherapy is a novel potential treatment for nicotine addiction. The aim of this study was to assess the safety and immunogenicity of a nicotine conjugate vaccine, NicVAX, and its effects on smoking behavior. Smokers (N = 68) were recruited for a noncessation treatment study and assigned to 1 of 3 doses of the nicotine vaccine (50, 100, or 200 microg) or placebo. They were injected on days 0, 28, 56, and 182 and monitored for a period of 38 weeks. Results showed that the nicotine vaccine was safe and well tolerated. Vaccine immunogenicity was dose-related (P < .001), with the highest dose eliciting antibody concentrations within the anticipated range of efficacy. There was no evidence of compensatory smoking or precipitation of nicotine withdrawal with the nicotine vaccine. The 30-day abstinence rate was significantly different across the 4 doses (P = .02), with the highest rate of abstinence occurring with 200 microg. The nicotine vaccine appears to be a promising medication for tobacco dependence.