There are no published prospective studies on the predictors of spontaneously quitting cigarette smoking in a nationally-representative U.S. population. This paper describes such a study, using a cohort taken from the First National Health and Nutrition Examination Survey (NHANES I, 1971-1975) and traced by the NHANES I Epidemiologic Followup Survey (1982-1984). "Successful" quitting (for at least 1 year at time of followup) was ascertained among 4779 adults (age 25-74 years) who were smokers at the time of NHANES I and not disabled at followup. Independent predictors of quitting (by proportional hazards multiple regression) were: (1) older age; (2) White race; (3) fewer cigarettes smoked/day; (4) higher household income; and (5) hospitalization in the followup period. Predictors of relapse (ex-smokers at NHANES I who were smoking again at time of followup) were: (1) younger age; (2) urban residence; and (3) female gender. These findings have implications for intervention strategies, public health projections and further research.