A 60 year old woman with chronic duodenal ulcer not responding to Cimetidine, was changed to Ranitidine. She had symptomatic improvement, but had bilateral breast engorgement and tenderness for which treatment was discontinued. A therapeutic trial on a second occasion had the same side effect which came on more rapidly and quickly. This complication in such severe form and recurrence on rechallenge requiring withdrawal of drug was observed for the first time with any H2 receptor antagonist.