The BK virus (BKV) belongs to the family of the polyoma group, which contains three species: JC, which is responsible for progressive multifocal leukoencephalopathy in acquired immunodeficiency syndrome (AIDS); simian virus 40 (SV40), which is a simian virus of little pathologic significance in humans; and BKV, which is usually not pathogenic and is found in the urine of asymptomatic individuals. Recently BKV has been reported to cause symptomatic infection in renal transplant patients. The authors report a rare case of a 14-year-old boy with AIDS who developed a BKV infection of the lung and kidney that progressed to diffuse alveolar damage and death. The infected type II pneumocytes in the lung and the tubular epithelial cells in the kidney showed large, homogenous purple intranuclear inclusions. The absence of necrosis and destruction made it possible to distinguish BKV infection from herpes simplex. The size of the infected cells and the lack of a halo around the nuclear inclusion helped rule out cytomegalovirus as the cause of infection. Electron microscopy detected the presence of 40-nm intranuclear viral particles compatible with BKV, and in situ hybridization established the diagnosis.