In the time period 1988-2000, a case-control study on large cell carcinoma of the lung was conducted in Montevideo, Uruguay. The study included 149 cases (139 men and 10 women) and 596 controls (556 men and 40 women) afflicted with diseases not related with tobacco smoking. Cases and controls were frequency matched on age, sex, residence and urban/rural status. According to our results, large cell carcinoma was strongly related with cigarette smoking (OR for heavy smokers 155.2, 95% CI: 30.6-786.1). Furthermore, all cases displayed higher risks for intensity compared with smoking duration. Smokers of black tobacco showed odds ratios (ORs) significantly higher than those presented by smokers of blond tobacco and smoking cessation displayed a reduction in risk of 95%, close to the risk of never smokers. Finally, filter use was not associated with any reduction in risk. It could be concluded that undifferentiated large cell carcinoma of the lung could be included among those lung tumors classified as Kreyberg I cancers.