A 65-year-old woman presented with a painful, swollen, red right thigh and the mild pain in the right abdomen without nausea, vomiting or diarrhoea that lasted for 1 week. Laboratory findings revealed elevated inflammatory markers. Computed tomography of the right thigh, abdomen and pelvis showed an abscess formation in the adductor muscles draining from the abscess that completely occupied the right retroperitoneum up to the diaphragm, dissecting downward through the inguinal canal. Appendix was enlarged with an appendicolith. Emergent exploratory laparotomy revealed a perforated appendix with psoas abscess. Pathohistological diagnosis revealed adenocarcinoma of the appendix. Thigh abscess is an uncommon condition with insidious clinical presentation. Therefore, early recognition and setting of the correct diagnosis enables adequate treatment avoiding additional complications and in some cases potential life-threatening conditions. When upper leg abscess is suspected or proven abdominal examination is mandatory.