The seroprevalence of human T-lymphotropic virus type 1 (HTLV-1) in Taiwan is 0.48%. In this study, we investigated the patterns of intrafamilial transmission of HTLV-1 in Taiwanese patients with adult T-cell leukemia/lymphoma (ATL/L) or tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM). Fifteen index patients (9 men, 6 women, aged 31-71 yr), 13 with ATL/L, and two with TSP/HAM, and 98 relatives were included. Of the 98 relatives, 23 were seropositive for HTLV-1. Spouses of 11 patients were studied. Seven of eight wives of male patients but none of the three husbands of female patients were HTLV-1 carriers. Mother-to-child transmission was found in seven of 13 families and in 15 of 75 children tested. The correlation of breast-feeding with seropositivity in two families with seropositive mothers indicates its important role in vertical transmission of HTLV-1. Our findings suggest that husband-to-wife and mother-to-child transmission are the main forms of intrafamilial transmission of HTLV-1 in Taiwan, a nonendemic area. Screening for HTLV-1 in family members of patients with ATL/L or TSP/HAM, and seropositive blood donors, may be warranted. Seropositive individuals should be educated to prevent the spread of the virus through sexual contact and breast feeding.