Whether the level of terminal deoxynucleotidyl transferase (TdT) activity in mononuclear cells from bone marrow and peripheral blood has prognostic significance has been analyzed prospectively in 164 children with T and non-T, non-B marked acute lymphoblastic leukemia (ALL). TdT was measured at diagnosis to assess its value as a predictor of duration of remission and length of survival. It was measured repeatedly during remission to assess whether it could predict relapse. Ninety-seven percent of the children achieved a complete remission of their disease, and 40% relapsed during the study. The level of TdT activity in blasts at diagnosis varied 1000-fold from patient to patient. There was no statistically significant relationship between TdT activity in cells at diagnosis and the achievement of complete remission, the duration of remission, or length of survival. TdT activity was significantly increased in the bone marrow of 65% of patients at the time of marrow morphological relapse, but was rarely increased in marrow from patients with isolated testicular or central nervous system relapse. Wide fluctuations in TdT activity were characteristically seen in mononuclear cells from the marrow and peripheral blood of patients with ALL at all stages of their disease. An isolated high value of TdT activity in the bone marrow or peripheral blood cannot be taken as evidence of impending relapse. Quantitative measurements of TdT activity alone on mononuclear cells from bone marrow and peripheral blood are helpful in differential diagnosis, but cannot guide therapy of children with ALL.