The concentration of oxalic acid was determined in the plasma of 15 patients with conservatively treated chronic renal insufficiency and 17 dialysis patients. A cumulation of oxalic acid was found in connection with uraemia. The extent to which plasma oxalic acid concentrations were raised depended on the degree of renal insufficiency and was directly related to the plasma creatinine values in all patients with or without dialysis. In the patients with chronic renal insufficiency the median plasma oxalic acid concentration was 74.4-18.5 (control group 27.0 +/- 7.4) mumol/l. In the dialysis patients the levels were even higher, at 137.5 +/- 56.0 mumol. By means of haemodialysis it was possible to lower the plasma oxalic levels by about the same amount as creatinine concentrations. The higher plasma oxalic acid concentrations seem to be an important pathogenetic factor in the formation of uraemic calcification in various organs. The therapeutic consequences are to increase the duration and frequency of dialysis and to remedy possible vitamin B6 deficiency.