We have studied lymphocyte subpopulations and the NK cytotoxicity of the PBMC from acute leukemia patients in complete remission after chemotherapy or ABMT and from normal donors. We have found a positive linear correlation between the percentage of subpopulations with CD3-CD16+, CD3-CD56+ and CD3-CD8+ phenotypes and the percentage of NK cytotoxicity. The slopes of the regression lines in the two groups of patients were lower than in normal donors, indicating a decreased ability of these cells to operate. The percentages of these subpopulations represent parameters for estimating the percentage of NK cytotoxicity both in normal donors and in patients with acute leukemia.