Diffuse alveolar hemorrhage (DAH) resulting from pulmonary capillaritis typically accompanies the systemic vasculitides and collagen vascular diseases. Isolated pulmonary capillaritis and DAH without systemic disease occurs in patients with antineutrophil cytoplasmic antibodies. However, isolated pulmonary capillaritis and DAH is not described for patients without clinical or serologic evidence for an underlying systemic disease. To describe such patients, we undertook a retrospective chart review of 29 patients with DAH and biopsy-proven pulmonary capillaritis from seven Denver hospitals. Eight (28%) were diagnosed with isolated pulmonary capillaritis without clinical, serologic, or histologic evidence of an associated illness. Their median age was 30 yr. No association with occupational or drug exposures was identified. All had lower respiratory tract symptoms; seven had upper respiratory tract symptoms. None demonstrated systemic disease or evidence of glomerulonephritis. All were antineutrophil cytoplasmic antibody negative. Other serologies were not significant where measured. Direct immunofluorescence studies of lung tissue were negative in five. Six presented with acute respiratory failure, four requiring mechanical ventilation. One died during initial hospitalization; seven survived. Median follow-up is 43 mo (7 to 73 mo). Five remain in remission. Two experienced multiple recurrences of DAH but without development of systemic disease while on therapy. Herein we characterize DAH and isolated pulmonary capillaritis in the absence of clinical, serologic, or histologic evidence indicating an accompanying systemic illness. The prognosis for this group appears favorable.