The purpose of this study was to determine the effect of adjuvant chemotherapy on serum lipids and lipoproteins in premenopausal nonmetastatic breast cancer patients. Fifty-nine premenopausal breast cancer patients were treated with adjuvant chemotherapy comprising cyclophosphamide, methotrexate, and 5-fluorouracil. Levels of serum FSH, LH, and estradiol; fasting plasma levels of total cholesterol, high density lipoprotein cholesterol (HDL), low density lipoprotein cholesterol (LDL), triglyceride, and apolipoprotein A-I, A-II, and B levels; and lipoprotein(a) [Lp(a)] were measured before treatment and 12 months after the beginning of chemotherapy. The changes in serum lipids after chemotherapy correlated significantly to changes in menstruation. Total cholesterol, LDL, and HDL cholesterol levels increased significantly only in patients with chemotherapy-induced ovarian dysfunction. Changes in apo B paralleled those in LDL and apo A-I and A-II HDL cholesterol. The serum concentration of Lp(a) increased significantly only in patients who developed permanent amenorrhea. There were no significant changes in triglyceride levels. Chemotherapy per se has no effect on serum lipids unless associated with induction of ovarian dysfunction. Ovarian failure after chemotherapy has an unfavorable effect on serum total and LDL cholesterol, apo B, and Lp(a) levels, but a favorable effect on HDL cholesterol.