We evaluated the effect of exogenously administered recombinant human granulocyte-macrophage colony stimulating factor (rHu GM-CSF) on plasma lipid and lipoprotein concentrations in 28 patients undergoing bone marrow transplantation (BMT). Twenty-one received rHu GM-CSF during the immediate post transplantation period (group 1) and seven did not (group 2). All patients received intravenous hyperalimentation starting at the immediate post-transplantation period until 3-5 days post engraftment. Plasma lipids and lipoproteins, liver and renal function tests and blood counts were determined prior to BMT (baseline levels) and during the immediate and late post transplantation periods. In both groups, marked changes of plasma total cholesterol (TC) and high-density lipoprotein cholesterol (HDL-C) concentrations were observed. During the immediate post transplantation period, TC levels decreased by 22.2% and 26.2% in groups 1 and 2, respectively. During the same period, HDL-C levels decreased by 41.4% and 37.5% in these two groups. At the late recovery phase TC and HDL-C resumed pre-treatment levels. These changes were in parallel to the fluctuations in total WBC counts. We conclude, therefore, that BMT has a significant transient effect on plasma lipids and lipoproteins. Although this response is unrelated to the exogenous administration of rHu GM-CSF it may be causally related to endogenous cytokines or other, yet unidentified, factors.