A 39-year-old woman presented to the Emergency Department with dysuria and persistent abdominal pain for several weeks. The pain increased with palpation. Physical examination was otherwise normal. Computerized tomography (CT) scan revealed a round structure of unknown origin against the caecum and terminal ilium suspect for a GIST tumor and a fluid-filled right uterine tube suggestive of pyosalpinx. The following day a laparoscopic surgery with removal of the pyosalpinx and complete right hemi-colon including the suspected lesion was performed and biopsies of the surrounding peritoneum, meso and ileum were taken. Immunohistochemical tests (CD117/DOG1) and molecular analysis (KIT/PDGFRA) ruled out GIST. SMA and desmin were also negative. β-catenin along with next generation sequencing which revealed a likely pathogenic mutation in exon 3 of the CTNNB1 gene, suggested a diagnosis of desmoid-type fibromatosis (DF). Biopsies of the lymph nodes were negative.