Gardner syndrome (GS), caused by mutations in the adenomatous polyposis coli (APC) gene, is characterized by polyposis coli, osteomas, and various soft-tissue tumors. If undetected or untreated, virtually all patients develop colonic carcinoma at a young age. Early detection, while essential, can be difficult because of attenuated phenotypes or spontaneous mutations. We present the clinicopathologic features of 11 identical fibromatous lesions that we have termed Gardner-associated fibroma (GAF), which not only appear to be a part of the spectrum of lesions associated with GS but, in some cases, represent the sentinel event leading to its detection. The GAFs occurred in 11 patients (5 boys and 6 girls; age range, 3 months–14 years), were solitary (n = 7) or multiple (n = 4), and occurred in the superficial and deep soft tissues of the paraspinal region (n = 7), back (n = 3), face (n = 2), scalp (n = 2), chest wall (n = 2), thigh (n = 1), neck (n = 1), and flank (n = 1). Histologically, GAFs resemble nuchal-type fibromas (NFs), consisting of thick, haphazardly arranged collagen bundles between which are found occasional bland fibroblasts, and having margins that frequently engulf surrounding structures including adjacent fat, muscle and nerves. After surgical excision, four patients developed recurrences that were classic desmoid fibromatoses (DFs). In one patient with multiple GAFs, one lesion had the features of GAF and DF in the absence of surgical trauma. A family history of GS or polyposis (n = 6) or DF (n = 1) was known at the time of surgery in seven patients. In three patients, the diagnosis of GAF resulted in the diagnosis of unsuspected APC in older family members, with the detection of an occult colonic adenocarcinoma in one parent. In the family of the remaining patient, no stigmata of GS were present. Genetic analysis of this child was performed to investigate the presence of a spontaneous (new) mutation; however, no abnormalities were detected. The significance of GAF is that it serves as a sentinel event for identifying GS kindreds, including those with a high risk for the development of DF, and it may potentially identify children with spontaneous mutations of the APC gene. Because NFs and GAFs resemble one another, we suggest that a subset of NF occurring in multiple sites, unusual locations, or children may be GAF.