We present the first case of metronidazole-related acute pancreatitis during a relapse of ulcerative colitis. A 31-year-old male patient, with inflammatory bowel disease on mesalamine treatment for the last 5 months, suffered from a 48-h abdominal pain and nausea. He was also administered metronidazole during a relapse 5 days before. Laboratory and imaging investigation revealed acute pancreatitis. Conservative measures and metronidazole as well as mesalamine withdrawal resulted in complete recovery. Clinical remission of ulcerative colitis was obtained by prednisolone administration. Mesalamine was reintroduced and no recurrence was noticed for a year. Acute pancreatitis was mainly attributed to metronidazole owing to the absence of recurrence after mesalamine readministration, the time of onset after the initiation of metronidazole and the lower typical range between its onset and mesalamine exposure. Identifying acute pancreatitis as a possible consequence of a certain medication in inflammatory bowel disease patients may be particularly important to determine further treatment of their disease.