Based on a clinicopathologic study of 34 patients with biopsy-confirmed diffuse pulmonary hemorrhage (DPH), we present an approach to the differential diagnosis of DPH with attention to histologic features such as capillaritis and the importance of laboratory tests such as anticytoplasmic autoantibodies (ACPA). The following DPH syndromes were encountered: antibasement membrane antibody (ABMA) disease (four cases); idiopathic pulmonary hemorrhage (four cases); Wegener's granulomatosis (WG) (five cases); probable WG (six cases); systemic necrotizing vasculitis otherwise unclassified (three cases); systemic lupus erythematosus (two cases); rheumatoid arthritis (one case); seronegative juvenile rheumatoid arthritis (one case); IgA nephropathy (one case); idiopathic glomerulonephritis (two cases--one with and one without immune complexes); and unclassified pulmonary-renal syndromes (five cases). Capillaritis was found in lung biopsy samples from 30 of the 34 patients (88%) and included patients with every type of DPH syndrome. Serologic testing for ACPA was useful in the diagnosis of WG. Identification of ABMA in the serum, kidney, or lung was the defining feature for the diagnosis of ABMA-mediated disease. Subclassification of the cases could not be done solely on histologic grounds except for cases of WG that showed granulomatous inflammation, foci of necrosis, or vasculitis. Classification of the remaining cases required correlation with (a) clinical and laboratory data; (b) biopsy samples from other sites such as the kidney, nasal sinuses, or skin; and (c) results of immunofluorescence or electron microscopy of kidney or lung biopsy samples.