Nine patients with the unusual combination of renal failure, nephrotic-range proteinuria, and biopsy-proved interstitial nephritis are described. Six of these patients had received nonsteroidal anti-inflammatory agents (three fenoprofen, one ibuprofen, one zomepirac, and one tolmetin). The remaining three patients had no history of exposure to drugs known to cause interstitial nephritis. Immunologic characterization of the infiltrating cells with monoclonal antibodies showed that the majority of cells in most cases were cytotoxic T cells, although some B cells were present in all cases. Giant collecting duct cells were seen in half the patients with drug exposure but in none of the others. Otherwise, there were no conspicuous morphologic differences between patients with and without drug exposure. Many of the patients had associated glomerular abnormalities. Only the zomepirac and tolmetin recipients showed pure interstitial disease. The three fenoprofen recipients and the zomepirac and tolmetin recipients regained normal renal function after the drug was discontinued. The combination of renal failure, nephrotic range proteinuria, and interstitial nephritis is one form of nephrotoxicity observed in patients treated with nonsteroidal anti-inflammatory agents. However, this lesion, which may be mediated by cytotoxic T cells, may also be seen rarely in patients with no apparent drug exposure.