We report a case of umbilical endometriosis externa and systematically review the literature regarding this finding. In our case report, a 47-year-old woman with cyclic umbilical bleeding, pelvic pain, and no previous umbilical surgery developed a spontaneous umbilical endometrioma, cured by surgical resection and bilateral salpingo-oophorectomy. In our review, 122 patients with documented umbilical endometriomas from 1966 to the present and 109 cases reported before 1953 were analyzed. Procedures used for diagnosis and/or therapeutic intervention included umbilical biopsy or resection, abdominal wall repair, diagnostic and/or operative laparoscopy, adhesiolysis, hysterectomy, and bilateral salpingo-oophorectomy. Variables included patient age; race; medical and surgical history; past use of oral contraceptives; history of umbilical pain, bleeding, or swelling; duration of signs and symptoms; size and color of the lesion; diagnostic evaluations; and medical or surgical management. Mean age of the study population was 37.7 +/- 0.98 years. Up to 40% of patients with extrapelvic endometriosis present with umbilical endometriomas, with symptoms occurring an average of 17.8 +/- 3.9 months before presentation. Lesions averaged 2.3 +/- 0.2 cm in diameter; were predominantly brown (19.1%), blue (13.2%), or purple (10.3%); and patients frequently had with pain (77.93%), cyclical bleeding (47.1%), and swelling (88.2%). Most patients had no history of endometriosis (73.1%), and laparoscopic, umbilical trocar-related seeding was identified in only 5 patients. Three patients received medical management, and surgical management was invariably curative, though 1 patient required repeat surgical therapy. Umbilical endometriosis is a common manifestation of external endometriosis, representing primary or secondary endometriosis, with a typical presentation that has little variation. Laparoscopic endometrioid tissue excision can result in iatrogenic seeding to the umbilicus. Historical and physical findings are pathognomonic, thus justifying a formal name--Villar's nodule, after the initial reporting physician. Surgical intervention is recommended, but medical therapy may result in long-term symptom control with minimal malignancy risk.