Acute respiratory failure caused by pulmonary vasculitis after L-tryptophan ingestion. This report describes two women who presented with severe respiratory failure and diffuse pulmonary infiltrates in the fall of 1989. Both required prolonged assisted ventilation because of severe shunt physiology. Open lung biopsies on admission revealed a small vessel vasculitis as the sole morphologic abnormality in both patients. Both responded to high dose corticosteroids. Neither patient exhibited evidence of systemic vasculitis, and neither had serologic evidence of an immune disorder. Common to both patients was ingestion of L-tryptophan. One patient exhibited several features of the eosinophilia-myalgia syndrome. The other patient did not appear to have the syndrome, but the temporal relationship between the onset of symptoms and initiation of L-tryptophan treatment was striking. The presentation of these patients alters our notions concerning the spectrum of clinical manifestations caused by this agent, and the response to methylprednisolone supports its efficacy in the treatment of this disorder. 