A case-control study of lung cancer was conducted in northwestern Germany in 1985-86. The study included 194 lung cancer cases and the same number of hospital controls and population controls who were matched to the cases by sex and age. Personal interviews were conducted by trained interviewers. We report here the effect of different smoking patterns--such as nonsmoking intervals, and time since quitting smoking--on lung cancer risk. Both quitting smoking and having a nonsmoking interval are seen to reduce lung cancer risk significantly. For a nonsmoking interval of three years or more, relative risk (RR) = 0.21, 95 percent confidence interval (CI) = 0.08-0.52; for quitting smoking for 10 years or more, RR = 0.23, CI = 0.11-0.48). A dose-response relationship was estimated for cigarette dose, length of nonsmoking interval, and time since stopped smoking.