Human T-lymphotropic virus type I (HTLV-I) carriers often have abnormal lymphocytes (Ably) that resemble malignant cells of adult T-cell leukemia (ATL). To identify predictors of the level of Ably in a longitudinal study of asymptomatic HTLV-I carriers, we analyzed data from 215 subjects (67 men and 148 women) with multiple Ably measurements on blood smears. Ably+ (those having Ably > 0.6% of leukocytes counted on a blood smear at least once) was strongly associated with a high proviral load (OR 8.9; 95% CI 4.1, 19.5). The association among those defined as Ably++ (Ably > 0.6% at all screens or Ably > 1.6% at least once) was higher (19.7; 6.9, 56.1). Ably++ was also significantly associated with male gender (2.8; 1.0, 7.8). Multivariate analysis of Ably level indicates that men with a high proviral load, high anti-HTLV-I titer and low anti-Tax reactivity have the highest Ably level.