Patients on hemodialysis (HD) are known to be at risk of carnitine deficiency. The aims of this study were to investigate the prevalence of carnitine deficiency in patients on dialysis and to compare the likelihood of a reduction in the serum carnitine level on HD with that on hemodiafiltration (HDF). The prevalence of carnitine deficiency, defined as a serum free carnitine level< 20 μmol/L, and that of carnitine insufficiency, defined as an acyl/free carnitine ratio> 0.4, was investigated in 150 patients on dialysis. The reduction rate of serum carnitine was then compared between HD and HDF. The prevalence of carnitine deficiency and that of carnitine insufficiency was 25.3 and 86.7%, respectively. Patients at high risk of carnitine deficiency accounted for 64.7%. Multivariate regression identified an association of duration of dialysis with the free serum carnitine level. The reduction rates of serum free carnitine in HD and HDF were 64±4 and 75±7%, respectively (p< 0.0001). The prevalence rates of carnitine deficiency and carnitine insufficiency were high in patients on dialysis. The serum carnitine reduction rate was greater with HDF than with HD.