A 35-year-old woman presented with a three-month history of left groin and thigh pain. Neurological examination and electromyography showed pathological features consistent with obturator nerve involvement. Imaging studies revealed a left retroperitoneal mass, which by pathological examination was shown to be metastatic adenocarcinoma of possible Miillerian origin. Primary tumor could not be detected in a follow-up period of three years. Obturator mononeuropathy can be the first manifestation of cancer. Cancer of unknown primary origin may occasionally be local, well-restricted and carry a good prognosis.