A high-performance liquid chromatographic method for the determination of 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid (furanic acid) and hippuric acid in human serum is described. Quantitative data were obtained from 20 blood donors, 26 non-dialysis patients and 41 dialysis patients. In healthy persons hippuric acid ranged from 0.2 to 0.6 mg/dl, furanic acid from 0.13 to 0.53 mg/dl. In dialysis patients the mean concentration of hippuric acid was elevated to 17.2 mg/dl (range 1.7-50.8 mg/dl) and the mean concentration of furanic acid was elevated to 1.89 mg/dl (range 0.17-6.45 mg/dl). In patients without renal insufficiency the concentrations were not elevated. These data are in accordance with previous data obtained by gas chromatographic methods. Preliminary results indicate that hippuric acid and furanic acid may be more specific parameters than other uremic retention products, and better indicators for the need for dialysis treatment than urea or creatinine.