The relation between smoking habits and the occurrence of spontaneous pneumothorax (SP) was studied in a Swedish population, predominantly urban. The study group consisted of a consecutive series of 138 patients admitted to Huddinge University Hospital over ten years for treatment of their first spontaneous pneumothorax. Their smoking habits were routinely analyzed. The sample consisted of 15,204 persons domiciled in the same circumscribed area (County of Stockholm). The annual incidence of first SP in the admission area is 6/100,000 for women and 18/100,000 for men. The study showed that smoking increased the relative risk of contracting a first spontaneous pneumothorax approximately ninefold among women and 22-fold among men and that there is a striking, statistically significant (p less than 0.001) dose-response relationship between smoking and the occurrence of SP. The life span risk of contracting SP among lifelong heavily smoking men is roughly estimated to be 12 percent but only 1/1,000 among never smokers.