In a population-based case/control study the differential lung cancer risk patterns due to tobacco smoking habits of various histological types have been investigated. The cases were 1432 deaths from lung cancer in the years 1980-1987, of which the histological type was known for 627 individuals. There was 54% squamous cell carcinoma, 24% small-cell carcinoma and 17% adenocarcinoma. Controls were 1343 deaths from other causes. Next-of-kin interviews were performed. The results of the study confirmed that cigarette smoking is associated with all histological types of lung cancer; however, the dose/response relationship between smoking and adenocarcinoma differed clearly from that observed in squamous and small-cell carcinomas. In the latter histological types the gradient of risk was much stronger as the number of cigarettes smoked or duration of smoking increased. The overall relative risk for smoking in small-cell and squamous cell carcinoma was 15.4 and 13.5 respectively, whereas that for adenocarcinoma was weaker (relative risk = 3.1). An interesting difference between squamous and small-cell carcinomas was found also for patients who gave up smoking. The effect of stopping was more pronounced in squamous cell carcinoma. The attributable risks for smoking in squamous and small-cell carcinoma were much higher (90% and 88% respectively) than for adenocarcinoma (64%). The data suggest that adenocarcinoma is likely to be related to other factors than tobacco smoking to a greater extent than are squamous or small cell carcinoma. Possible sources of bias, such as missing histological diagnoses, are discussed in detail.