To clarify the relationship between Strongyloides stercoralis, infection with human T cell lymphotropic virus type-1 (HTLV-1), and serum immunoglobulin E (IgE) levels, epidemiologic investigations of these two infections were conducted in inhabitants of Okinawa, a subtropical zone in Japan. Blood and feces samples were taken from 1,347 healthy inhabitants (554 males and 793 females). Antibody to HTLV-1 was measured by particle agglutination, enzyme-linked immunosorbent assay, and Western blotting. The presence of Strongyloides was determined by direct detection of rhabditiform larvae in fresh stool on agar-plate cultures. Serum IgE levels in 127 inhabitants were measured by a fluoroenzyme immunoassay. Antibody to HTLV-1 was detected in 23.0% of the blood samples and was more frequent in females (25.1%) than in males (20.0%) (P < 0.05). Strongyloides were detected in 21.9% of the feces samples and were more frequent in males (31.9%) than in females (14.9%) (P < 0.001). The prevalence of both infections increased with age, especially in persons 50 years of age and older: The prevalence of Strongyloides infection was significantly higher in HTLV-1 carriers (31.6%) than in those without HTLV-1 infection (P < 0.001). The level of IgE was low in HTLV-1 carriers, and significantly lower in HTLV-1 carriers than in noncarriers among inhabitants with Strongyloides infection. Both HTLV-1 and Strongyloides infections are endemic in the area studied.