We measured serum lipoprotein(a) [Lp (a)] concentrations in 50 uremic patients treated on continuous ambulatory peritoneal dialysis (CAPD) and compared them with those in 29 uremic patients on hemodialysis (HD) and those in 62 normal controls. The median values were 47.9 mg/dl in CAPD patients, 25.2 mg/dl in HD patients, and 11.7 mg/dl in controls, respectively. These differences were statistically significant when assessed by Kruskal-Wallis test (p < 0.0001). Thirty-five out of 50 patients on CAPD (70%) and 12 out of 29 patients on HD (41%) had Lp(a) concentrations above 30 mg/dl, whereas these high values were observed in only 15% of normal controls. This difference in prevalence of high Lp(a) was also significant by 2 x 3 chi-square test (p < 0.01). There was a significant positive correlation between Lp(a) and apolipoprotein B (r = 0.517, p < 0.0001). In CAPD patients, 9 with ischemic heart disease had a significantly higher median Lp(a) than those without it (67.4 vs 40.9 mg/dl, p < 0.01 by Mann-Whitney U-test). These results suggest that high levels of serum Lp(a) might contribute to an increased risk for ischemic heart disease in CAPD patients, and that there may be a relationship between Lp(a) and apolipoprotein B metabolism in CAPD patients.