Adult T-cell leukemia/lymphoma (ATL) and human T-cell lymphotropic virus type I-associated myelopathy/tropical spastic paraparesis (HAM/TSP) are associated with differing patterns of immune dysfunction. Biomarkers of immune activation may correlate with perturbations of immune function associated with these diseases. We conducted a pilot cross-sectional study to assess four candidate biomarkers of immune activation. beta 2-microglobulin, neopterin, tryptophan, and kynurenine levels were assayed in stored sera from asymptomatic, human T-cell leukemia virus type I (HTL V-I)-seronegative (HTLV-I-) and HTLV-I-seropositive (HTLV-I+) individuals, and ATL and HAM/TSP patients previously enrolled in seroepidemiological studies in Jamaica. Mean levels of beta 2-microglobulin, neopterin, and kynurenine were significantly elevated among ATL patients compared to the other study groups. Mean tryptophan levels were significantly lower among ATL and HAM/TSP patients than HTLV-I- and HTLV-I+ groups. No significant differences in biomarkers were found between the HTLV-I- and HTLV-I+ groups. Among HAM/TSP patients, a significant association was found between elevated neopterin levels and symptoms of less than 4 years duration. In Cox proportional hazards regression modeling, neopterin and tryptophan were found to be independent predictors of survival among ATL patients. This study demonstrates a differential pattern of biomarkers of immune activation among ATL and HAM/TSP patients compared to HTLV-I- and HTLV-I+ individuals. Neopterin and tryptophan may be useful clinical indicators of disease severity and prognosis among HAM/TSP and ATL patients.