Of 46 patients with acute crescentic glomerulonephritis involving 20 to 90% of glomeruli, 16 had no definable systemic disease and no significant glomerular immune deposits by immunofluorescent or electron microscopy. Anti-GBM antibody and circulating immune complexes were further excluded by radioimmunoassay and Raji cell assay in all patients tested. Clinical features included a 10:6 male:female ratio, mean age of 58 years (range, 13-77), disease duration of less than 3 months, rapidly deteriorating renal function, and frequent pulmonary manifestations. Nine patients had oliguria, serum creatinine concentrations over 6 mg/100 ml, and required dialysis, but three of these patients subsequently recovered renal function. These three patients and seven patients with creatinine concentrations of less than 6 mg/100 ml have not progressed to chronic renal failure. In this series, idiopathic acute crescentic glomerulonephritis without immune deposits was more common than was immune complex or anti-GBM nephritis. The clinical, laboratory, and pathologic characteristics of these patients were similar to those reported in anti-GBM and immune-complex-induced glomerulonephritis. These observations expand the spectrum of rapidly progressive crescentic glomerulonephritis. They suggest that glomerular immune deposits may be less important than other factors in determining the extent of renal injury and subsequent clinical course in crescentic glomerulonephritis.