A case of auranofin(AF)-induced acute interstitial pneumonitis in a patient with rheumatoid arthritis (RA) is reported and possible mechanism of the incidence is discussed. A 57 year-old woman with a history of RA starting in early April 1989, had been prescribed AF for seven days prior to her referral to this hospital. Because of the severity of her polyarthritis, AF was discontinued; therapy with gold sodium thiomalate (GST) instead was started on 23 June 1989. The patient responded quite favorably. At a total cumulative dose of 160mg of GST, however, she developed generalized exfoliative dermatitis and the drug was discontinued. On 29 Sept. 1989 when her skin lesions cleared up completely, she was put on AF again. Six days after resumption of AF therapy, she developed fever. Nonproductive cough and dyspnea on exertion was observed within the following three days. The chest X-ray film revealed diffuse reticular-linear infiltrates more prominently in both upper lobes. Arterial blood gas studies showed hypoxemia and a restrictive ventilatory defect was demonstrated by pulmonary function studies. AF was stopped. High dose pulse iv corticosteroid(CS) therapy combined with oral CS was instituted. Immediate improvement of both subjective and objective findings was observed. By using the patient's peripheral blood lymphocytes, delayed lymphocyte stimulation test(DLST) against the probable causative agents was performed before the bolus CS therapy. Stimulation Index(shown in percentage) was markedly elevated in AF(724%) as compared with GST(163%). The fiberoptic bronchoscopy with bronchoalveolar lavage(BAL) was performed. Analysis of BAL fluid revealed increased cellularity (9 x 10(5)/ml) with marked lymphocytosis of 79%.(ABSTRACT TRUNCATED AT 250 WORDS)