In hemodialysis (HD) and continuous ambulatory peritoneal dialysis (CAPD) patients, an increase in plasma lipid, lipoprotein or apolipoprotein is considered to be a serious risk factor for atherosclerosis. Recently, epidemiological studies have shown that a high serum lipoprotein (a) (Lp(a)) concentration is an independent risk factor for the development of cardiovascular diseases, especially in CAPD patients. In the present study, we investigated alterations of serum Lp(a) levels, before and 1, 3 and 6 months after starting CAPD. The mean serum Lp(a) level (+/- 1SD) in 45 CAPD patients was 33.6 +/- 12.6 mg/dl. The serum Lp (a) and LDL levels were decreased one month after starting CAPD. The serum levels of Lp(a), LDL, apolipoprotein AI, AII, B, CII and E were increased significantly 3 and 6 months after starting CAPD. There was a significant correlation between the serum levels of Lp(a) and LDL (r = 0.45, p < 0.05) or lipoprotein B (r = 0.73, p < 0.001), and serum albumin (r = 0.62, p < 0.001). However, there was no significant difference in serum Lp(a) levels between cases with low and high glucose concentrations in the peritoneal effluents. The preliminary study showed that the peritoneal effluents contained a large amount of Lp(a). It appears that these changes are caused by excretion of Lp(a) into the peritoneal effluents and/or by Lp(a) hyperproduction in the liver of CAPD patients.