We studied serum lipid and lipoprotein changes occurring during chemotherapy in 57 patients with chemosensitive cancers, including 18 malignant lymphomas, 18 breast carcinomas, 14 small-cell lung carcinomas, and 7 urothelial-cell carcinomas. Patients who responded favorably to chemotherapy demonstrated a significant increase in serum total cholesterol and LDL cholesterol values, with the singular exception of breast-cancer patients, who exhibited a nonsignificant decrease in both of these parameters. Serum levels of free cholesterol and HDL cholesterol did not show any significant changes. Finally, serum triglycerides tended to increase after effective chemotherapy, but this was of statistical significance only in breast-cancer patients. Although our findings were based on a rather small number of patients, they indicate that the lipid and lipoprotein disorders reported in cancer patients are reversible by effective treatment of the tumor, suggesting that these disorders are a secondary phenomenon of malignancy.