Plasma homocysteine and plasma and erythrocyte folate concentrations before and after hemodialysis were measured in 31 patients with ESRD. Homocysteine and folate were measured by HPLC-fluorometric and microbiological methods, respectively. The mean plasma homocysteine level declined from 36.8 to 24.2 mumol/L during hemodialysis, indicating that homocysteine can be partly removed by hemodialysis (P < 0.0001). Mean plasma folate concentration before hemodialysis was 46.4 nmol/L and decreased to 25.9 nmol/L after hemodialysis (P < 0.0001), whereas mean erythrocyte folate concentration did not change (1295 and 1385 nmol/L before and after hemodialysis, respectively). Plasma folate concentrations showed a significant negative correlation with homocysteine concentrations before and after hemodialysis (r = -0.53, P < 0.003, and r = -0.59, P < 0.001, respectively). Furthermore, there were significant negative correlations between plasma homocysteine and erythrocyte folate concentrations both before (r = -0.60, P < 0.0005) and after hemodialysis (r = -0.49, P < 0.005). All patients had homocysteine concentrations over 12.0 mumol/L before hemodialysis, and only three had homocysteine concentrations lower than 12.0 mumol/L after hemodialysis. Although significant correlations existed between homocysteine and folate concentrations, the majority of the patients in this study appeared to have adequate folate nutriture as assessed by blood folate concentrations. It remains to be determined whether patients with ESRD have an altered homocysteine metabolism.