The effects of surgical menopause on lipoprotein levels and their time course were studied in 31 premenopausal women who were undergoing hysterectomy and bilateral oophorectomy for non-malignant conditions. Lipoprotein levels were measured before oophorectomy and afterwards at 6 and 12 weeks, then at intervals of 3 months for 18 months. Low density lipoprotein (LDL) cholesterol levels rose significantly (P less than 0.05) in the 6 weeks after operation from a mean of 3.57 (SD 0.66) mmol/l to 4.21 (SD 0.84) mmol/l with no significant changes thereafter. There were no significant changes in cholesterol in the other density fractions or in triglyceride levels. High density lipoprotein (HDL) subfractions were measured in 10 of the women to assess any change in the relative amounts of cholesterol carried on HDL2 and HDL3, since the protective effect of HDL is believed to be conferred by the HDL2 fraction only. No significant change was found in either fraction. The increase in LDL cholesterol would be expected to result in an appreciable increase in the risk of developing coronary heart disease, but cannot wholly account for the increase in cardiovascular disease associated with oophorectomy.