Renal disease was found at necropsy in 27 of 60 catheterized baboons (Papio cynocephalus). Primary diagnoses were infarcts in eight baboons, septic embolic nephritis in four, and mesangioproliferative glomerulonephritis in 15. Infarcts and septic embolic nephritis are multifocal renal diseases; mesangioproliferative glomerulonephritis is diffuse, progressive and often accompanied by severe proteinuria, hypoalbuminemia (less than 3 g/dl), and generalized edema. Five of the 15 baboons with glomerulonephritis also developed uremia and died of renal failure. Ten baboons with glomerulonephritis were given microbiological examination during their clinical course. Bacteria were isolated from the peripheral blood of eight baboons and from the catheter itself in two. The organisms isolated (in decreasing order of frequency) were Herellea sp., Streptococcus sp., Klebsiella sp., Staphylococcus sp., and Providencia sp. Prospective immunofluorescence studies were done on frozen sections from six catheterized baboons with glomerulonephritis and two uncatheterized controls. There were granular deposits of IgG in all six, IgM in five, C3 in four, and IgA and C4 in two. In the controls, only trace amounts of IgM were seen in one animal. The amount of IgG deposited in th glomeruli correlated with the severity of the glomerulonephritis. Bacterial antigens were found by immunofluorescence in glomeruli of three of five baboons examined. Glomerulonephritis in these baboons seems to be immunologically mediated, and in origin, pathogenesis and morphologic expression resembles shunt nephritis in humans with infected indwelling shunts and catheters.