Gardner fibroma represents a rare and recently described soft tissue tumor entity in children and young adults. It consists of haphazardly arranged coarse and hyalinized collagen fibers combined with loosely arranged bland spindle and fibroblastic cells. The case of a 13-year-old male patient with Gardner fibroma and osteoma and multicentric desmoid type fibromatosis in his mother is presented with detection of a (heterozygotic) germline mutation of the APC gene leading to a de novo stop codon (deletion of base pairs 5033-5036). FISH analysis revealed a structural loss of heterozygosity (LOH) in the APC gene on chromosomal locus 5q21 in one out of five analysed desmoids of the mother, no LOH of APC gene in the Gardner fibroma. Gardner fibroma in children and young adults may serve as an indicator lesion for familial adenomatous polyposis (FAP), Gardner syndrome, a familial desmoid type fibromatosis without other manifestations of APC or a new APC gene mutation. For the clinician, this diagnosis should be commented upon accordingly by the surgical pathologist. As the result of a detected APC gene mutation, continuous follow-up for the development of colorectal tumors and desmoid type fibromatosis as well as a familial screening for FAP is recommended.