Altered immunologic reconstitution is observed in patients treated with high-dose chemotherapy consisting of cyclophosphamide, cisplatin, and carmustine, melphalan, or etoposide with autologous bone marrow support, and it is similar to that seen in patients treated with high-dose chemoradiotherapy and allogeneic bone marrow transplantation. A decrease in the absolute number and percentage of B cell and CD4 antigen-positive cells and an increase in the absolute number and percentage of CDS and la antigen-positive cells occur along with a decrease in the CD4/CD8 ratio that persists for 6–12 months after high-dose chemotherapy and autologous bone marrow support. These changes have been associated with four serious infectious episodes usually seen only in immunocompromised patients. The above changes were not seen in patients treated with high-dose busulfan, a drug that has relatively specific effects on granulocytes. It is postulated that these alterations result from effects of chemotherapy on the residual lymphocytes or on the environment of repopulating lymphocytes. Functional studies of lymphocyte populations during immunologic reconstitution after standard-dose combination chemotherapy and high-dose chemotherapy with autologous bone marrow support are needed.