An 87-year-old woman on oral prednisolone was diagnosed with a cholecystoduodenal fistula (CDF) caused by a cytomegalovirus-associated duodenal ulcer (DU) and was managed conservatively. A CDF caused by a DU is extremely rare. Although surgical repair is recommended for the treatment of a CDF caused by cholecystolithiasis, appropriate treatment for CDF caused by a DU remains controversial. This case report of a CDF caused by a DU suggests that conservative treatment is feasible in the absence of DU-associated complications, such as an untreatable hemorrhage or obstruction; this finding is compatible with previously reported cases that were conservatively treated.