Serum free and esterified carnitine levels as well as lipids were investigated in patients undergoing regular hemodialysis (HD) treatment before and during 12 weeks of treatment with L-carnitine (1 g i.v.) at the end of each HD. The results were compared with those obtained in patients on continuous ambulatory peritoneal dialysis (CAPD; n = 15) or intermittent peritoneal dialysis (IPD; n = 3) and healthy controls (CO; n = 20). In HD patients (n = 23) total carnitine (TC) was 49.9 ± 3.9 (CO: 46.0 ± 2.5; NS), free carnitine (FC) was 31.6 ± 2.8 (CO: 37.4 ± 1.3; p < 0.05), short-chain acylcarnitine (SCC) was 17.0 ± 1.8 (CO: 7.2 ± 0.9; p < 0.0001) and long-chain acylcarnitine (LCC) was 1.2 ± 0.2 μmol/l (CO: 0.6 ± 0.1; p < 0.05). FC was in the normal range in CAPD (35.6 ± 3.2) and IPD (44.5 ± 8.0 μmol/l) patients, whereas SCC (30.1 ± 3.5) and LCC (2.9 ± 0.2) levels were maximal elevated in IPD patients (11.8 ± 0.8 and 1.5 ± 0.2 on CAPD). Therefore, TC was higher in IPD than in CAPD patients (77.5 ± 5.0 vs. 49.0 ± 3.5 μmol/l). 12 weeks after L-carnitine supplementation in HD patients, TC was 313.9 ± 22.6, FC was 207.7 ± 12.4, SCC was 99.6 ± 12.1 and LCC was 7.1 ± 0.6 μmol/l. TC and FC were significantly lower in females compared with males. Total cholesterol and ketone bodies were normal, HDL cholesterol was significantly decreased before and after L-carnitine supplementation. On the other hand, a paradoxical rise in serum triglycerides was observed. Our data indicate abnormalities of free and esterified carnitine in HD, CAPD and IPD patients. The markedly elevated serum levels for TC, FC, SCC and LCC following L-carnitine therapy suggest limited carnitine utilization in chronic uremic patients.