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Most patients with anorectal abscess are diagnosed clinically based on pain, erythema, warmth, and fluctuance. Some patients, however, present with subtle or atypical signs. CT is easily accessible and is commonly used for diagnosis and delineation of anorectal abscess. The purpose of this study is to determine the sensitivity of CT scan in detecting perirectal abscesses and to see if immune status impacts the accuracy of CT. A retrospective study was conducted to identify patients from 2000 to 2009 with International Classification of Diseases, 9th Revision code 566 (anal or rectal abscess). Patients included had a CT scan less than 48 hours before drainage. Patients with CT-positive abscess were compared with patients with CT-negative abscess. Patients were categorized as either immunocompetent or immunosuppressed based on documentation of diabetes mellitus, cancer, human immunodeficiency virus, or end-stage renal disease. One hundred thirteen patients were included in this study. Seventy-four (65.5%) were male and the average age was 47 years. Eighty-seven of 113 (77%) patients were positive on CT for anorectal abscess. Sixty of 113 (53%) patients included in this study were immunocompromised. CT missed 26 of 113 (23%) patients with confirmed perirectal abscess. Eighteen (69%) of these patients were immunocompromised compared with CT-positive patients (42 [48%], P = 0.05). The overall sensitivity of CT in identifying abscess was 77 per cent. CT lacks sensitivity in detecting perirectal abscess, particularly in the immunocompromised patient.