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.