A multidisciplinary methodology was applied to study 6 patients with acute leukemia (AL) following treatment for Hodgkin's disease. All 6 patients developed acute monocytic leukemia according to cytochemical criteria; morphologically, 3 cases were undifferentiated monoblastic leukemia; 3 cases were partially differentiated monocytic leukemia. In 3 of the 6 cases, cell suspensions from peripheral blood and/or bone marrow bore predominantly surface receptors for the Fc fragment of IgG and/or C-3 fraction of complement and exhibited variable degrees of phagocytosis. Two cases lacked any recognizable membrane markers or phagocytic activity. Terminal transferase enzymatic activity was not diagnostically elevated. Soft agar bone marrow culture studies revealed no growth or a characteristic leukemic cluster pattern. Chromosome analysis done in 1 of the 2 patients studied revealed hypodiploidy and structural rearrangement. Prospective studies, done serially, will help determine whether development of AL in patients with Hodgkin's disease results from the influence of intensive and potentially oncogenic therapy or represents progression from a preleukemic stage inherent to the original tumor.