We have analyzed the distribution and prognostic significance of terminal deoxynucleotidyl transferase (TdT) and adenosine deaminase (ADA) in connection with conventional cytology, cytogenetics, response to therapy, and survival. The study population consisted of 78 patients with AML, 44 patients with Ph1 + CML in chronic phase, and 35 adult patients with Ph1 + CML in blastic phase, among which 5 cases presented as Ph1 + acute leukemia. Nine percent of the AML cases were positive for TdT and were characterized by a high percentage of blast cells in bone marrow, myeloid features by cytochemistry and absence of the Philadelphia chromosome. The median ADA values of the TdT+ AML cases were several times higher than those obtained for the TdT- cases. The survival calculated for the two groups of AML cases subdivided according to ADA levels was significantly longer (p less than 0.025) for the patients with low levels of ADA (less than 250 U/10(8) cells). In chronic phase of CML, TdT was absent and ADA values were increased over normal controls only in cases with early signs of transformation. In blastic phase, 31% of the 35 cases were positive for TdT, and ADA values were significantly higher (p less than 0.001) in TdT+ than TdT- cases. The survival calculated from the onset of transformation was significantly longer for the TdT+ acute phase (10.4 mo) compared to the TdT- patients (4.8 mo; p less than 0.025). Four cases presenting as Ph1 + acute leukemia were TdT+ and had elevated levels of ADA; 3 of them responded to ALL therapy, reverting to a stable phase of CML.