Serum deoxythymidine kinase (TK) was measured in 15 patients with the acute type of adult T-cell leukemia (ATL), in 4 with chronic ATL, in 10 with lymphoma type ATL, in 9 with pre-ATL, in 11 with human T-cell leukemia virus type I (HTLV-I) associated with myelopathy (HAM) and in 19 HTLV-I carriers. All these patients were positive for anti-HTLV antibody. The level of TK in pretreatment serum was highest in acute ATL (15.6-1600 U/l, median 107 U/l). It was elevated in chronic ATL (5.4-55.0 U/l, median 37.6 U/l) and lymphoma ATL (6.8-316 U/l, median 16.8 U/l) but normal in pre-ATL (1.8-4.7 U/l, median 2.8 U/l), HAM (1.2-6.0 U/l, median 3.0 U/l) and HTLV-I carriers (1.1-4.6 U/l, median 2.3 U/l). Statistical examination revealed a significant difference between the levels of acute ATL and chronic ATL/lymphoma ATL. In the patients of this series, a close correlation between the level of TK and lactic dehydrogenase (LDH) was statistically present (p less than 0.01). These facts indicate that TK level is a useful indicator of the aggressiveness of ATL cells.