We have surveyed the incidence of adult T-cell leukemia/lymphoma (ATLL) in an endemic area of 290,464 inhabitants for 7 years. We now revise our previous results on the basis of additional findings and estimate the age- and sex-specific cumulative rate for HTLV-I carriers, the adoption of which is recommended by current cancer epidemiology as a new age-standardized incidence rate. An unequivocal age-dependent increase in seroprevalence was observed for both sexes with a characteristic predominance in females. The age-dependent seroconversion in females may be partly explained by additional infection from infected husbands to their wives but the reason for men remains obscure. The mean annual number of incident cases of ATLL was 11.4, giving 3.9 ATLL patients annually per 10(5) inhabitants, 6.1 per 10(5) inhabitants aged over 30, and 85.0 per 10(5) seropositives aged over 30. Crude annual incidence rate of ATLL among 10(5) male seropositives aged over 30 was 145.3 and that for females was 55.2 and 95% confidence intervals of ATLL incidence rates were 34.8 to 255.7 for males and 6.4 to 104.1 for females, respectively. Although the sex ratio of 80 ATLL patients was 1.35, males are more prone to the disease (46 male patients among 4,522 male seropositives aged over 30 vs 34 female patients among 8,801 female seropositives aged over 30; p less than 0.001) for unknown reason(s). Morbidity in male seropositives aged over 30 is 2.6 times as high as that of females. Decennial incidence rates in males in their fifties and sixties were significantly higher than those in females. The remarkable male preponderance in oncogenicity of HTLV-I may be due to the fact that men are more prone to the disease and the number of female carriers in the denominator used to calculate the incidence rate is larger than that of males. The whole life span (0-79) cumulative risk for males was 6.9% and significantly higher than that of females (2.95%).