Pleural effusion suddenly appeared in a 35-year-old caucasoid woman. Thoracentesis removed 300 cc. of creamy fluid which had all the physical and chemical characteristics of chyle. No treatment except bed rest was administered. Complete resorption of the remainder of the fluid in the chest ensued, and the patient made an uneventful recovery. The most common causes of chylothorax are neoplasm and trauma. In this patient there was no indication of any underlying malignancy and no history of trauma was obtained except extremely mild exercise performed the day before the onset of symptoms. It seems likely that on rare occasions minimal injury may cause chylothorax, which may take a mild clinical course and go unrecognized unless thoracentesis is performed.