While endometriosis, defined as the presence of endometrial tissue in extrauterine sites, is most frequently encountered within the peritoneal cavity, a small but significant proportion of cases occur at extra-abdominal soft tissue sites, particularly in relation to previous abdominal surgery. We reviewed the cases of endometriosis of soft tissue sites seen at a tertiary soft tissue center. All cases of extra-abdominal soft tissue endometriosis diagnosed at this institution over a 13-year period were reviewed, and clinical and pathologic findings were recorded. Forty-five patients had diagnoses of soft tissue endometriosis and there were 34 diagnostic biopsies and 26 surgical excision specimens. All but 1 case were abdominal wall lesions, with 1 located in the upper arm. A total of 33 patients presented with lesions in scars of previous operations (31 in Pfannenstiel incisions for Caesarean sections, presenting with a median interval of 6 years (range 1-16 years) following surgery). The lesions ranged in size from 1 to 8 cm (median 3.5 cm). One case showed decidualized stroma with trophoblast cells, while 2 had secondary adenocarcinoma arising from endometriosis. Eighteen cases were tested for β-catenin expression immunohistochemically, of which 5 showed at least focal nuclear positivity in the surrounding fibrous tissue (although not within glands or stroma). Soft tissue endometriosis is seen most commonly in surgical scars, particularly following Caesarean sections. Spontaneous endometriosis also most commonly occurs in the abdominal wall, although can occur exceptionally at unusual sites, such as extremities. Secondary changes, including carcinomas, can arise from endometriosis. The differential diagnosis of these lesions includes fibromatosis, which may be erroneously diagnosed on small, nonrepresentative core biopsy specimens.