Multiple, controlled clinical trials support the efficacy of nortriptyline as a smoking cessation agent. Although therapeutic plasma nortriptyline concentrations (PNCs) are known for the treatment of depression, little is known about PNCs in smoking cessation treatment. PNCs from three randomized, placebo-controlled smoking cessation trials (N=244) were analyzed both separately and pooled. PNCs normalized for dose and weight were associated with cigarettes per day and race, but not with sex or age. Greater smoking was associated with decreased normalized PNCs. In addition, both Asian and black populations had significantly higher normalized PNCs than the white populations. Weak and inconsistent associations between PNCs and self-reported side effects were observed. PNCs were linearly related to end of treatment and long-term biochemically verified smoking abstinence. Maximum therapeutic effects were observed over a range of plasma concentrations somewhat lower than those found effective for the treatment of depression.