Bone marrow cells were cultured in liquid media with and without phytohemagglutinin (PHA) to test bone marrow lymphocyte response during the remission period of acute myelogenous leukemia (AML). Mitogen-stimulated cultures from nine leukemic patients while in complete remission showed high percentages of morphologically transformed lymphocytes, but proliferation of these cells was significantly decreased (p less than 0.02) in cultures from six patients who subsequently relapsed (mean remission duration 10 months) and died. In contrast, lymphocyte proliferation in cultures from three AML patients with long remission (greater than 30 months) was comparable to controls. Parallel cultures without added PHA showed a progressive decrease in total viable cell number with time, but an increasing percentage of macrophages in both control and AML cultures. These studies suggest that bone marrow lymphocyte proliferation, but not morphologic transformation, is impaired in some patients with AML, and that identification of this defect may be of prognostic value.