Increases in mesangial cell number may herald glomerular scarring, but they are not irreversible. This study sought mechanisms by which surplus glomerular mesangial cells can be cleared. A small proportion of cultured mesangial cells exhibited typical morphological features of apoptosis (programmed cell death), which was increased by growth factor deprivation or exposure to cycloheximide, stimuli known to increase apoptosis in other cell types. Apoptosis was confirmed by typical internucleosomal chromatin cleavage. In vivo, clear morphological evidence of mesangial apoptosis leading to phagocytosis by neighboring mesangial cells was obtained in self-limited mesangial proliferation induced in rats by Thy1.1 antibody, apoptosis occurring approximately 10-fold more frequently than in the healthy rat glomerulus. Indeed, changes in glomerular cell number in Thy1.1 nephritis strongly suggested that apoptosis is the major cell clearance mechanism counterbalancing cell division, thereby mediating resolution of glomerular hypercellularity in experimental mesangial proliferation.