In a series of thirty-six patients treated with clozapine we report four cases who developed upper gastrointestinal symptoms suggestive of reflux oesophagitis within 6 weeks of starting this drug. Subsequent endoscopic examination revealed moderately severe erosive oesophagitis in three of them. Of these one had received treatment for a peptic ulcer in the past but the other two had no previous history of any upper gastrointestinal disorder. The pharmacology of clozapine in relation to gastro-oesophageal reflux is discussed.