BACKGROUND: The characteristics of patients who develop a fistula-in-ano after an anorectal abscess are unclear. OBJECTIVE: Our study explored this relationship and patient factors associated with fistula development. DESIGN: International Classification of Diseases, 10 Revision, and Classification of Interventions and Procedures, version 4, codes were used to identify all of the patients with a primary anorectal abscess. Multivariable analysis was used to identify factors predictive of fistula formation. SETTINGS: The study was conducted in a district general hospital. PATIENTS: Patients with anorectal abscess who were admitted to our institution (2004-2015) were included. MAIN OUTCOMES MEASURES: The rate of subsequent fistula formation was measured. RESULTS: A total of 1970 abscess patients were identified; 70.0% (n = 1379) were men, and 7.3% (n = 144) had Crohn's disease. Fistulas occurred in 16.2% (n = 319) at a median of 7 months (interquartile range, 3-7 mo). Patients with Crohn's disease were more than twice as likely to develop a fistula than patients without Crohn's disease (32.6% vs 14.9%; OR = 2.5 (95% CI, 1.7-3.7); p < 0.001). Patients with Crohn's disease with a fistula were more likely to be women (55.3% vs 34.6%; p = 0.007) and aged <30 years (51.1% vs 24.3%; p < 0.001) versus patients without Crohn's disease with a fistula. At multivariable analysis of the entire cohort, male sex (OR = 0.7 (95% CI, 0.5-0.9); p = 0.005) and diabetes mellitus (OR = 0.5 (95% CI, 0.3-0.9); p = 0.027) were associated with a reduced likelihood of developing a fistula after abscess formation. LIMITATIONS: The study was limited by its single-center scope, retrospective analysis, and lack of an established definition for Crohn's disease. CONCLUSIONS: Abscesses are more common in men, but progression to fistula is more likely in women. The rate of fistula progression in Crohn's disease is twice that in patients without Crohn's disease. Identification of patients at risk may help delineate those who will benefit from a more conservative surgical approach, enhanced follow-up, or investigation after abscess drainage. See Video Abstract at http://links.lww.com/DCR/A798.