Twenty-nine psoriasis patients on 5 mg/kg cyclosporin A (CyA) therapy were studied for 3 months using the furosemide test. Five of them (17%) showed an abnormal renal acidification capacity after furosemide administration: The urinary pH did not sink under 5.3 after furosemide, while the ammonium and titrable acid levels were significantly low. There were no significant differences from controls regarding the serum potassium or fractional potassium excretion. Nevertheless, the transtubular potassium gradient was lower in patients with an abnormal furosemide test result. We conclude that some patients treated with a low dose CyA therapy developed an abnormality in the distal tubular acidification.