OBJECTIVE To determine the incidence of the risk factors for developing diarrhea in horses after celiotomy for colic. METHODS Retrospective cohort study. METHODS 357 adult horses that had celiotomy for colic at the teaching hospital between Jan 1, 1990 and Sep 1, 1994. METHODS Medical records of horses that had celiotomy for colic were reviewed to abstract information regarding development of diarrhea, signalment, history, and treatment. RESULTS In horses that had celiotomy for colic, the incidence of diarrhea was 53.2% (190/357). Using multiple logistic regression, horses with a disorder of the large intestine were approximately twice as likely to develop diarrhea after celiotomy as horses that had surgery for other types of intestinal lesions (P < 0.001). Even after accounting for the effects of large intestinal surgery, horses that also had an enterotomy were approximately 1.5 times as likely to develop diarrhea (P = 0.042). Diarrhea in horses associated with duration > 2 days, isolation of Salmonella spp from feces, or leukopenia was categorized as being severe. Incidence of severe diarrhea was 27.5% (98/357). Using multiple logistic regression, horses that had surgery of the large intestine were approximately 2.5 times as likely to develop severe diarrhea after celiotomy as horses that had surgery for other types of intestinal lesions (P = 0.006). In horses that had celiotomy for colic, those that were fed grass hay were approximately half as likely to develop severe diarrhea as were horses that were not fed grass hay (P = 0.018). CONCLUSIONS Although the risk factors identified for the development of diarrhea are not alterable, knowledge of them will enable clinicians to better advise clients and to better prepare for medical management of horses after surgery.