The histologic diagnosis of acute tubular necrosis (ATN) frequently poses considerable difficulty, especially in its early phases. This is particularly true in postmortem material in which autolysis is commonly present. This study was designed to assess the value of enhanced autofluorescence in the diagnosis of ATN. The method has previously been shown to be useful in the identification of myocardial infarcts in both humans and experimental animals. We studied 12 patients with a histologic and clinical diagnosis of ATN. In all, severe hypotension of diverse etiology was detected from 15 hours to seven days prior to death. In all cases, a bright yellow autofluorescence was observed in the necrotic tubules in ordinary H & E sections. In addition, enhanced autofluorescence was observed in the necrotic tubular epithelium in the unstained, paraffin-embedded sections. Nine kidney sections from five medico-legal autopsy cases undergoing autolysis from 11 to 48 hours after death were used as controls. These kidneys invariably exhibited a faint dull olive green fluorescence, quite different from the fluorescence of the necrotic tubules. No effort was made to distinguish between ischemic (tubulorrhexis) and toxic (tubulonecrosis) changes by the fluorescent method. The mechanism of the enhanced autofluorescence is not clear. We conclude that ultraviolet light examination of H & E kidney sections allows accurate recognition of ATN.