To find a clue to lung cancer etiology in Japan, differences in the pattern of lung cancer histology and related time trends between Osaka, Japan, and the North West Region of England were investigated. Material comprised all incident lung cancer cases registered in both regional registries (14,521 in the Osaka Cancer Registry and 29,859 in the North West Regional Cancer Registry). (1) The age-standardized incidence rate of lung cancer was higher in the North West Region than in Osaka (80.4 among males and 20.9 among females per 100,000 population in 1979-82 versus 32.1 and 9.2 respectively). (2) A higher proportion of adenocarcinoma was observed in Osaka (36.3% in males and 62.0% in females) than in the North West Region (12.3% and 18.9% respectively). (3) Using the relative frequencies of each histological type according to sex and age-group, age-standardized incidence rates were calculated for the main lung cancer histological types. It was shown that the incidence rates of adenocarcinoma were similar in the two areas (10.6 in males and 5.3 in females in Osaka versus 10.0 and 3.5 in the North West Region, respectively) while those of squamous cell and small cell carcinomas were much higher in the North West Region than in Osaka. (4) Time trends of incidence rates showed an increase only for adeno- and small cell carcinomas in Osaka. Slight increases were observed for adenocarcinoma in both sexes and for squamous cell carcinoma in females in the North West Region. (5) Considering cigarette consumption and the relative risks of smoking in the two areas, the possible existence of other risk factors for adenocarcinoma in both sexes in Japan, besides active smoking, was suggested.