A 43-year-old man was referred to our institution with lower abdominal pain and perineal swelling and fullness of more than a year. At the referring hospital, he underwent a nondiagnostic computed tomography–guided biopsy of what was presumed to be a large pilonidal cyst. His medical history was unremarkable for hematochezia or alteration in bowel function. Magnetic resonance imaging at our institution revealed a 14 × 10.5 × 25-cm bilobed, cystic mass situated in his pelvic outlet in the retrorectal space, markedly compressing his rectum (Figure 1and Figure 2). There was a suggestion of internal hemorrhagic vs proteinaceous debris with degenerative changes within the mass and enhancing mural nodules. The mass extended through the floor of the pelvis and into the perineal area. It did not appear to invade his rectum or involve the regional vasculature.