It is not clear whether tubular cell necrosis is present or not in acute renal failure (ARF) of ischaemic type ("acute tubular necrosis"). In order to get quantitative data, using precisely defined criteria for tubular cell necrosis, 25 renal biopsies from 24 patients with ARF (11 obtained in the active phase, 14 in the early recovery period) were compared with 12 control biopsies. In all 1959 proximal cells and 1603 distal cells were analysed by electron microscopy. Cellular disintegration was very rare in all groups. Shrinkage necrosis (apoptosis) was not present in the proximal tubules of the controls and was rare in ARF (1.6-2.1%). In the distal tubules of controls 2.7% of all cells showed shrinkage necrosis. The incidence in ARF was not significantly increased. "Non-replacement sites" in distal tubules (probably loci where cells have recently been desquamated) were significantly increased in number (5.2%) in the active phase in ARF compared to controls and recovery. The relative number of regenerating cells was not increased. These data show that there is no widespread necrosis of tubular cells in ARF. The increased incidence in distal tubules of focal, denuded areas of the basement membrane in the active phase of ARF indicates a slightly increased desquamation of cells and/or a failure to cover such sites by adjacent cells. This process is not restricted to the brief induction phase of ARF but continues during the whole active phase.