Introduction: Although commonly associated with rectal bleeding, hemorrhoids may thrombose and cause intense anal pain and swelling. Previous studies of hemorrhoids have been limited by retrospective design and lack of diagnosis by experienced colorectal surgeons. Even less well studied are risk factors associated with thrombosed hemorrhoids. In this study, we prospectively assessed risk factors for thrombosed hemorrhoids in a cohort of patients evaluated by colorectal surgeons. Methods: Based on a comprehensive review of prior published risk factors, we compiled a questionnaire encompassing over 80 factors potentially related to the development of hemorrhoids. This questionnaire was completed by patients presenting with hemorrhoidal symptoms to the offices of five board-certified colorectal surgeons in an urban setting. The incidence of these factors in patients with thrombosed hemorrhoids was compared to patients with nonthrombosed hemorrhoids. Univariate analysis using chi square for discrete variables and Student’s t-test for continuous variables was performed. Variables with a p value <0.1 were then tested by multiple stepwise logisitic regression.Results: The entire study cohort consisted of 212 patients (56% male; 86% Caucasian). Forty patients (19%) had thrombosed hemorrhoids and 172 patients (81%) had other forms of hemorrhoid disease. There was a significantly (p=0.02) higher incidence of swelling in patients with thrombosed hemorrhoids (90%) compared to patients with nonthrombosed hemorrhoids (50%). Univariate analysis revealed differences between the thrombosed hemorrhoid and nonthrombosed hemorrhoid patient groups in percentage of individuals under 40 years of age (42% vs. 24%, p=0.01), proportion of patients spending longer than ten minutes on the toilet during defecation (6% vs. 13%, p=0.04), anal sex (15% vs. 6%, p=0.06), and the presence of frequent abdominal bloating (20% vs. 34%, p=0.07). On multivariate analysis, age younger than 40 years (p=0.02; odds ratio=2.4) and anal sex (p=0.047, odds ratio=3.1) independently predicted thrombosed hemorrhoids.Conclusions: This first reported prospective study of patients examined by a group of experienced colorectal surgeons revealed swelling as the primary complaint of patients with thrombosed hemorrhoids. Independent risk factors for thrombosed hemorrhoids include age younger than 40 and a history of anal sex.