Peripheral blood samples from 55 previously untreated leukemia patients (33 males, 22 females) were analysed for terminal deoxynucleotidyl transferase (TdT) activity. TdT was significantly higher in patients with acute myelogenous leukemia (AML; P < 0.001), chronic myelogenous leukemia (CML; P < 0.05) and acute lymphoblastic leukemia (ALL; P < 0.001) when compared with controls. One patient with chronic lymphocytic leukemia (CLL) had undetectable TdT. Among leukemic patients, ALL patients had higher concentration of TdT than CML and AML patients. Females had higher TdT activity than males, although the difference between the two groups was not statistically significant. 68% TdT+ and 32% TdT- patients were in blastic crisis. Patients with more than 10% of blasts in the circulation had significantly higher TdT than blast-negative patients (P < 0.001). No difference in survival was observed between TdT+ and TdT- group. From these results, we conclude that the absolute TdT concentration is of little prognostic value in leukemia patients.