Thirty-seven patients with adult T-cell leukemia had serum beta 2-microglobulin (beta 2M) estimated at presentation. A significant association was found between beta 2M and the subtypes of the disease: beta 2M in the acute type is significantly higher than in the chronic and smoldering types (p less than 0.01), and beta 2M in the lymphoma type is apparently higher than in the chronic and smoldering types, although the difference between the lymphoma and chronic types was not statistically significant. An apparent reduction in beta 2M levels after chemotherapy was observed in 7 patients who reached partial or complete remission. One patient who was resistant to chemotherapy showed elevated beta 2M values even after intensive chemotherapy. In another patient, who attained partial remission with a concordant reduction in beta 2M, renewed elevation of beta 2M was observed in parallel with exacerbation of other parameters such as lactate dehydrogenase (LDH), WBC, calcium and clinical manifestations. Correlation analysis was carried out between beta 2M, LDH, calcium, lymphoid cell count and the grade of clinical severity. The beta 2M level was moderately correlated with LDH, calcium, the grade of clinical severity but not with the lymphoid cell count. The clinical severity also correlated with LDH and calcium but to a lesser extent than with beta 2M. These data support the use of the beta 2M in the evaluation of adult T-cell leukemia.