Predictors of smoking behavior change were examined in a randomized controlled trial of individualized smoking cessation counseling delivered by a smoking cessation counselor during periodic health examination. Self-reports of not smoking at 6 and 18 months and attempts to quit were greater, but not significantly so, in the intervention group compared with the usual care group. There was no difference between the intervention group and the usual care group in reported continuous abstinence. Multivariate analysis showed that longer periods of abstinence in the past and having smoking identified as the main problem were important predictors of subsequent quitting. Having fewer other smokers in the household, stronger intentions to stop smoking in the next month, and being in the intervention group were also significant predictors of abstinence at 6 months, but not at 18 months. Those who had tried to quit by 6 months and 18 months were more likely to be in the intervention group, to have greater motivation to stop smoking, and to have more problems of daily living. Supplementing physician's advice with individualized smoking cessation counseling during health maintenance examinations was associated with a greater short-term quit rate and more quit attempts over 18 months than physician advice alone, but did not influence continuous abstinence from cigarettes over this time.