Urinary acidification after ammonium chloride loading was evaluated in 22 patients treated with amphotericin B and in ten normal volunteers. Six patients were studied while receiving amphotericin B; all had defective renal excretion of acid, although systemic acidosis was absent. Four of these patients were restudied following treatment; three were normal at two, four, and eight months respectively; and one remained abnormal at ten months. Three patients, initially studied one to five months after completion of treatment, excreted an acid load normally. Fourteen patients studied more than a year following treatment with amphotericin B were all normal. Incomplete renal tubular acidosis is a common, but reversible, complication of amphotericin B therapy. Urinary acidification is normal within a few months after treatment is discontinued. Alkali therapy is indicated in patients who develop renal tubular acidosis and potassium supplementation should be given if hypokalemia appears.