HTLV-I is sexually transmitted more efficiently from men to women than vice versa, and the majority of HTLV-I endemic areas report a female preponderance of HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP) cases. The objective of this study was to estimate the gender- and age-specific incidence rates of HAM/TSP in the general population as well as in the HTLV-I-infected population in Jamaica and in Trinidad and Tobago. Incidence rates for HAM/TSP were computed based on all reported incident cases in both countries between 1990 and 1994. Population census reports for 1990 were used to calculate the population at risk. The age-standardized HAM/TSP incidence rate (mean +/- standard error of the mean) in Jamaica was 1.8 +/- 0.2/100,000 person years (PY). Among individuals of African descent in Trinidad and Tobago, the rate was 1.7 +/- 0.4/100,000 PY. As in HTLV-I seroprevalence, the incidence rate of HAM/TSP increased with age through the fifth decade of life and was three times as high in women than in men. The HAM/TSP incidence rate, calculated as a function of the number of HTLV-I-infected persons in each age stratum, is higher in women (24.7/100,000 PY) than in men (17.3/100,000 PY). With HTLV-I infection, the lifetime risk of developing HAM/TSP was estimated to be 1.9% overall and is slightly higher in women (1.8%) than in men (1.3%). Thus, the higher prevalence of HTLV-I in women in endemic areas does not fully explain the preponderance of female HAM/TSP, suggesting that other cofactors must be present. The higher incidence rate in women between the ages of 40 and 59 years, as well as the increase in HAM/TSP incidence rates with age, are indicative of the importance of adult-acquired HTLV-I infection, presumably through sexual transmission.