Human T-lymphotropic virus type I (HTLV-I) causes HTLV-I-associated myelopathy/tropical spastic paraparesis and adult T cell leukemia in a small percentage of infected individuals. HTLV-I infection is increasingly associated with clinical manifestations. To determine the prevalence of clinical manifestations in HTLV-I infected individuals, we conducted a cross-sectional study of 115 HTLV-I-infected blood donors without myelopathy and 115 age- and sex-matched seronegative controls. Subjects answered a standardized questionnaire and underwent physical examination. Compared with controls, HTLV-I-infected subjects were more likely to report arm or leg weakness (OR = 3.8, 95% CI: 1.4-10.2; OR = 4.0, 95% CI: 1.6-9.8, respectively), hand or foot numbness (OR = 2.1, 95% CI: 1.1-3.9; OR = 4.8, 95% CI: 2.0-11.7, respectively), arthralgia (OR = 3.3, 95% CI: 1.7-6.4), nocturia (OR = 2.7, 95% CI: 1.04-6.8), erectile dysfunction (OR = 4.0, 95% CI: 1.6-9.8), and to have gingivitis (OR = 3.8, 95% CI: 1.8-7.9), periodontitis (OR = 10.0, 95% CI: 2.3-42.8), and dry oral mucosa (OR = 7.5, 95% CI: 1.7-32.8). HTLV-I infection is associated with a variety of clinical manifestations, which may occur in patients who have not developed myelopathy.