AbstractAppendiceal collision tumours are extremely rare, with most reported cases describing tumours consisting of a mucinous component and a neuroendocrine component. Low-grade appendiceal mucinous neoplasms, in some cases, have a tendency to rupture and disseminate their mucin-producing cells throughout the abdominal cavity, leading to a clinical syndrome known as pseudomyxoma peritonei (PMP). We present the case of a 64-year-old male who initially presented with acute appendicitis and was subsequently found to have PMP and appendiceal malignancy. After several years of scans, surgical intervention and histological analysis, it became apparent that the appendiceal malignancy was comprised of distinct cell types. The patient underwent two rounds of cytoreductive surgery with hyperthermic intraperitoneal chemotherapy, which resulted in a 2-year disease-free period. Unfortunately, the PMP recurred, having morphological changes consistent with a more aggressive disease process.