Sirolimus inhibits human fibroblast cell proliferation in cell cultures from transbronchial biopsies of lung transplant recipients. However, a few cases of interstitial pneumonitis and bronchiolitis obliterans organizing pneumonia have been recently described in solid organ transplant recipients, including a fatality in a heart transplant recipient. We studied the patterns of pulmonary adverse effects associated with sirolimus in 4 renal transplant recipients who developed pulmonary opacities on chest radiograph, which were proved to be noninfectious in origin. Lung biopsy was performed to obtain histological diagnosis (3 interstitial pneumonitis, 1 necrotizing vasculitis). Symptoms were dyspnea (4), cough (2), hemoptysis (1), fever (1) and eyelid edema (1). Those with interstitial pneumonitis had bilateral basal opacities on chest X-ray, and histopathology showed mild lymphoplasmocytic interstitial inflammation, scattered intraalveolar epitheloid granulomas and a focal pattern of organizing pneumonia. Serum C-reactive protein (CRP) was elevated and bronchoalveolar lavage revealed lymphocytosis (77, 79.5 and 31%). The fourth patient had an opacity localized in the upper lobe, which progressed to both the lower lobes, and histopathology showed multifocal necroses of lung tissue with lymphoplasmocytic vasculitis and scattered granulomas. In this patient, the serum CRP level was not elevated and bronchoalveolar lavage was normal. Pulmonary symptoms and opacities on chest radiograph resolved and the serum CRP level became normal after sirolimus was stopped in all patients. Sirolimus may be a cause of interstitial pneumonitis or pulmonary vasculitis, and withdrawal of sirolimus is therapeutic.