A 17% frequency of cytologically negative pericardial effusion (CNPE), accompanied in some cases by tamponade, occurred a median of 12.6 months from the onset of treatment for localized small-cell carcinoma of the lung. CNPE was apparently caused by toxicity of radiation/chemotherapy treatment rather than recurrent cancer. The occurrence of CNPE does not appear to represent enhanced toxicity of immediate (as opposed to delayed) concurrent chemoradiotherapy, but may be a consequence of the superior survival status of patients treated in this way. The onset of chest pain and/or dyspnea associated with increase in cardiac silhouette and positive echocardiogram allowed accurate diagnosis. Each instance was relatively easily managed by catheter drainage, and, for some patients, with the addition of nonsteroidal antiinflammatory drugs. It is important to recognize the possibility that a radiation/chemotherapy-related syndrome of pericardial effusion/tamponade may occur so that early diagnosis can be made and the risk of fatal tamponade avoided.