Rituximab, a chimeric monoclonal antibody against CD20, very rarely causes lung toxicity. Clinical presentations include lung infiltrates, alveolar hemorrhage, and adult respiratory distress syndrome. Three cases of organizing pneumoinia (formerly called bronchiolitis obliterans with organizing pneumonia or BOOP) have been reported. In our experience, organizing pneumonia occurred in 2 of 25 patients treated with rituximab, for RA and Castleman's disease, respectively. Because organizing pneumonia may be asymptomatic, as illustrated by one of our cases, we recommend obtaining a chest radiograph routinely before rituximab re-treatment.