Interstitial nephritis may be associated with a variety of auto-immune disorders, but there have been no reports among these of primary biliary cirrhosis. A patient presenting with sodium- and potassium-losing nephropathy due to interstitial nephritis was discovered to have primary biliary cirrhosis which was asymptomatic. Correction of the electrolyte abnormalities with sodium and potassium supplements had no effect on renal function, but creatinine clearance increased from 28 to 68 ml/minute during a seven-week course of prednisolone. The occurrence in primary biliary cirrhosis of other forms of renal tubular dysfunction, the frequent presence in both interstitial nephritis and primary biliary cirrhosis of multi-system involvement and the close temporal relationship in this patient, suggest that the two conditions were related.