Low dose pulse methotrexate (MTX) has become a widely used therapy for rheumatoid arthritis (RA) because of its good response rate profile. With the increased use of MTX, reports of opportunistic infections associated with MTX therapy have appeared. Fourteen cases of pneumocystis carinii (PC) pneumonia in patients receiving low dose MTX have been previously reported. Yet, no case of PC pneumonia associated with low dose MTX has so far been reported in Japan. We report the first case in Japan of PC pneumonia occurring in a patient with rheumatoid vasculitis who was receiving low dose MTX. A 70-year old woman with 13 year history of RA presented with 3-day history of rapidly aggravating dyspnea, dry cough and fever. She had been receiving MTX 7.5 mg/week for 2.5 months because of her vasculitis symptoms. She had also been receiving prednisolone 7.5 mg/day which had been successfully tapered from an initial dose of 15 mg/day. At the time of her presentation with respiratory symptoms, all of her vasculitis symptoms had been alleviated. A chest radiograph revealed diffuse interstitial shadowing bilaterally and bilateral hilar and right lower lung field infiltrates. Her arterial blood gas showed severe hypoxemia (PaO2 27.7 torr). Polymerase chain reaction assay of bronchoalveolar lavage fluid showed PC. Although the patient required ventilatory support for 9 days, she was successfully treated with trimethoprime-sulphamethoxazole and methylprednisolone pulse therapy. Eight months later, the patient was well with no evidence of vasculitis or respiratory symptoms.