A woman complained of pain resembling sciatica which recurred over a period of more than 4 years in a certain time-relation to the menstrual cycle. An exploratory operation revealed small dark nodules adhering to the sciatic nerve. Excision of the nodules and subsequent irradiation were followed by complete disappearance of the cyclic pain in the leg. The operation also disclosed the presence of a peritoneal invagination, drawn downward toward the greater sciatic notch, in the posterior part of the pelvis. This "pocket sign" led to the discovery of the offending lesions in the present case, and may help to locate similar hidden lesions in other cases of endometriosis.