Fifteen patients with chronic active hepatitis (CAH) were tested in order to ascertain the site of tubular dysfunction in renal tubular acidosis (RTA) associated with CAH. Renal plasma flow and GFR were reduced in the patients compared to controls (P < 0.005). Underbasal and acidification conditions, twelve patients showed normal acid-base balance and net acid excretion, while three patients had basal hyperchloremic metabolic acidosis while passing alkaline urine and showing bicarbonaturia. A sustained acidification test showed adequate urinary acidification in these three patients. Bicarbonate loading carried out in two of the three patients showed a proximal tubular acidifying defect (Type 2 RTA), Distal RTA (Type 1 RTA) complicating CAH is widely known, but these data suggest that CAH can also involve the proximal convoluted tubule in isolation.