Background: Carnitine deficiency is common in patients on dialysis. Serum free carnitine concentration is significantly lower in patients on hemodialysis (HD) than in healthy individuals. However, there are few reports on serum free carnitine concentration in patients on peritoneal dialysis (PD). Methods: We examined serum concentrations of total, free, and acylcarnitine and the acylcarnitine/free carnitine ratio in 34 PD and 34 age-, sex-, and dialysis duration-matched HD patients. We investigated the prevalence of carnitine deficiency and clinical factors associated with carnitine deficiency in the PD group. Results: Prevalence of carnitine deficiency was 8.8% in the PD group and 14.7% in the HD group (P = 0.45). High risk of carnitine deficiency was found in 79.4% of the PD group and 85.3% of the HD group (P = 0.52). Carnitine insufficiency was found in 82.3% of the PD group and 88.2% of HD group (P = 0.49). Multivariate analysis revealed that duration of dialysis and age were independent predictors of serum free carnitine level in the PD group. Conclusions: The prevalence of carnitine deficiency, high risk of carnitine deficiency, and carnitine insufficiency in PD patients was 8.8%, 79.4%, and 82.3%, respectively. These rates were comparable to those in patients on HD.