We report a case of pulmonary alveolar proteinosis (PAP) and cytomegalovirus (CMV) infection occurring in a 5-year-old boy with acute lymphoblastic leukemia. The association of PAP and CMV infection is rare, but the possible etiologic role of CMV in the production of PAP is raised. Pulmonary alveolar proteinosis is being reported with increasing frequency in immunocompromised patients, and this disease may be difficult to distinguish from other causes of diffuse lung disease both clinically and radiologically. Pulmonary alveolar proteinosis, either alone or in combination with CMV infection, should therefore be considered in the differential diagnosis of diffuse lung disease in these cases.