Light, immunofluorescence and electron microscopic studies were carried out on renal biopsies from 32 randomly selected adult monkeys (Macaca irus). Histopathology was limited to glomeruli and consisted of mild to moderate segmental increases in mesangial cells, mesangial matrix, and/or glomerular basement membrane (GBM) thickness in 41% of the animals. Granular deposits of IgM were present in the mesangial region and along the GBM in 72% of the monkeys, whereas IgG, C1q, C4 and C3 were detected in approximately 30%. Electrondense deposits were seen predominantly in epithelial foot processes adjacent to the GBM and, to a lesser extent, in the mesangium. Those monkeys with the heaviest IgM deposition were found to have decreased serum levels of C3, IgM and IgA. Follow-up biopsies over a period of 3 to 11 months revealed that the disease process was persistent yet nonprogressive. No correlation with age or sex was noted. All animals examined were clinically healthy and had normal renal function. This is the first documented occurrence of spontaneous immune-complex glomerulonephritis in a large monkey population. It appears to be a persistent disease which does not progress to renal insufficiency and which may serve as an investigative model for mild nonprogressive forms of human glomerulonephritis.