Nine patients with biopsy-proven acute interstitial nephritis (AIN) were analyzed for their urinary indices, determined on the basis of the first urinary sample upon admission, before any therapeutic intervention. Isosthenuria was present in all patients with a mean urinary osmolality of 283 +/- 48 mOsm/l and a urine/plasma osmolality ratio of 0.9 +/- 0.1. Urinary sodium was more than 40 mEq/l in 8 out of the 9 patients studied. The other urinary indices studied were indicative for both prerenal and intrinsic renal disease.