A comparison was made between cigarette smoking histories of 31 women below age 50 years who had a diagnosis of lung cancer on hospital discharge and smoking histories of 124 women below age 50 years who had been hospitalized for other conditions. Of the women with lung cancer, 28 (90%) were current or former cigarette smokers; 72 (58%) of the comparison women were smokers. The relative risk estimate for lung cancer among smokers as compared with nonsmokers is 6.7, with 90% confidence limits of 4.0 and 11. Risk of lung cancer increased with the amount that the women smoked. The smokers with lung cancer had been smoking for longer periods than the smokers with other conditions. Assuming that the association is causal, cigarette smoking was responsible for about 77% of the lung cancer of young women in this survey.