Only a few pathologic reports exist describing adult onset Still's disease (AOSD) with pulmonary involvement. We report this very rare case of AOSD complicated with cryptogenic organizing pneumonia (COP). A 32-year-old woman was referred with high spiking fever, salmon-pink rash in her arms and legs, and polyarthralgia. The laboratory data showed marked increases in white blood cell count, an erythrocyte sedimentation rate, and C reactive protein, ferritin, and liver dysfunction. All cultures remained negative, as were autoantibodies and rheumatoid factor. The patient was strongly suspected of AOSD according to specific diagnostic criteria. However, chest X ray disclosed an infiltrative shadow accompanied by air bronchogram in the upper lobe of the right lung and therapy with antibiotics was initiated. As the patient did not respond to antibiotics and a remittent fever of over 38°C, a flexible bronchoscopy was performed. Organizing pneumonia was diagnosed by transbronchial lung biopsy (TBLB) histology and radiologically, and the lesions were thought to be due to pulmonary involvement of AOSD. Therefore, she was diagnosed with AOSD complicated with COP. Oral treatment with prednisolone (30 mg/day) resulted in rapid disappearance of the infiltrative shadow. Symptoms and markers of inflammation also improved. Clinicians should be aware that COP can be a complication of AOSD.