The two most common diseases of captive owl monkeys (Aotus trivirgatus) are hemolytic anemia and glomerulonephritis. The anemia is characterized by total red blood cell counts between 0.45 and 3.44 x 10(6) microliters, hemoglobin values as low as 1.0 g/dl, and many circulating nucleated red blood cells. Centrilobular necrosis in the liver, extramedullary hematopoiesis in liver and spleen, and hemoglobin casts in kidney tubules are prominent histologic features. Hemosiderin and lipofuscin often are found in liver, spleen, kidney and lymph nodes. Microthrombi and microinfarcts sometimes are scattered throughout the brain. Glomerular lesions in Aotus have been described previously and are characterized by increased numbers of mesangial cells and matrix, glomerulosclerosis and electron dense deposits in basement membranes. Lymphocytes, plasma cells and eosinophils frequently are present in the interstitium. In the early stages the cellular infiltrate is periglomerular. The foci then grow to encompass adjacent glomeruli and tubules. Finally, large portions of the kidney are affected and connective tissue proliferates. The incidence of extramedullary hematopoiesis in the liver correlated significantly with that of interstitial nephritis (0.001 less than p less than 0.01) but not with glomerular lesions. The two kidney lesions, glomerulonephritis and interstitial nephritis correlated strongly in incidence. They also found with equal frequency in 87 monkeys with clinical evidence of anemia. This analysis indicates that there may be no common pathogenesis of the hematologic and renal abnormalities as seen in certain autoimmune diseases. However, there could be complex interactions between two or more disease mechanisms that account for the various manifestations of disease.