Reduction of the risk of lung cancer as a result of giving up smoking is examined according to the number of years of cessation from smoking and the number of cigarettes that were smoked per day before quitting smoking. Patients with histologically diagnosed lung cancer from 26 hospitals in six cities in the US were compared with controls matched for age, sex, race, time of diagnosis, and hospital. Smoking habits were recorded by trained interviewers using a questionnaire. In men, a fairly consistent reduction in risk with years of cessation for each category of cigarettes per day before giving up smoking was found. In women, however, a much less consistent pattern was observed. Analysis of the data by histologic type of lung cancer showed that among women, as among men, risk was less and declined more consistently in those with Kreyberg I cancers than in those with Kreyberg II tumors. The inconsistency was seen mainly in patients with Kreyberg II cancers, which were more common among women.