Blood samples, 2259 in winter and 523 in other seasons of the year, were collected nationwide in Japan from inhabitants (primarily farmers) in areas with no known man-made pollution, and analyzed for cadmium. The levels were distributed log normally, and were lower among young adults and increased gradually to reach a plateau at the 40-59 age group, where the values in females (about 3.6 ng/ml as a geometric mean) were significantly higher than in males (3.0-3.4 ng/ml). The sex difference was positive (P less than 0.01) even when 77 pairs of levels were compared between husbands and their wives, both being nonsmokers in the age range of 40-59 years. Smoking habits gave an additional increase in the blood cadmium level. The increase was dose dependent up to 20-29 cigarettes/day and leveled off with further consumption. Effects of passive smoking could not be confirmed. Seasonal variation in blood cadmium level appeared negligible. Variation in the level by geographic location in the country was of doubtful significance. The estimated ratio of cadmium doses by two routes, i.e., via the gastrointestinal tract and via the lungs, was in agreement with the ratio of the blood cadmium level among nonsmokers and the additional increase in the level due to smoking.