Bacillus Calmette-Guerin(BCG) has been widely used for the prophylaxis of superficial bladder tumor recurrence and for the treatment of bladder carcinoma in situ. More than 95% of patients who receive BCG instillation tolerate the treatment well and side reactions have been reported in less than 5% of patients. Most side effects are minor and self-limiting. However, a rare occurrence of severe systemic reactions have been reported. Among the severe systemic reactions, hypersensitivity pneumonitis should be considered in patients with pneumonic complications after BCG instillation in cases where the culture for mycobacteria is negative in the sputwn, brochoalveolar lavage and blood specimen. In addition, a fiberoptic bronchoscopy with transbronchial lung biopsy demonstrates a fibrosis of the alveolar septums, where there is and an increased lymphocyte count with out tuberculous inflammatory changes, the and CD4 : CD8 ratio is increased and no symptomatic response to antituberculosis chemotherapy is observed. Here we report a 68 years old man with interstitial pneumonitis following intravesical BCG instillation.