We studied serum lipid and lipoprotein changes before and after induction treatment in 25 acute nonlymphocytic leukemia (ANLL) and in 18 acute lymphocytic leukemia (ALL) patients in order to investigate their relationship with disease activity and their prognostic relevance. ANLL at diagnosis is associated with significantly low levels of all lipid parameters, the same applies to ALL patients apart from plasma triglycerides and very-low-density-lipoprotein cholesterol (VLDL-C) which are significantly higher than in the normal population. In ANLL responders, after effective chemotherapy, a significant increase of total cholesterol, low-density-lipoprotein cholesterol (LDL-C) and apolipoprotein B levels, without changes of high-density-lipoprotein cholesterol (HDL-C) values, is observed. A further decrease of total cholesterol and LDL-C was found in nonresponders and in ANLL responders treated with granulocyte-macrophage colony-stimulating factor (GM-CSF), known for its cholesterol-lowering action; in fact after the completion of GM-CSF therapy, these parameters returned progressively toward normal values. In ALL responders an increase of total cholesterol, HDL-C and apolipoprotein A1 with a simultaneous decrease of triglycerides and VLDL-C is evident; no variation was found in the nonresponder group. These results suggest a close correlation between serum lipids and acute leukemia: total cholesterol and LDL-C in ANLL, and HDL-C and VLDL-C in ALL may be considered reliable markers of complete remission and may be useful in the follow-up of leukemic patients.