Clinical and radiologic findings in a 73-year-old man who developed a systemic illness while receiving intravesical bacillus Calmette-Guérin (BCG) immunotherapy for bladder cancer are presented. Thin-section chest computed tomographic findings included a diffuse pattern of small nodules consistent with miliary disease. Potential mechanisms explaining the pulmonary disease resulting from intravesical BCG treatment include a hypersensitivity reaction or actual BCG infection of the lungs.