Mallory-Weiss lesions were considered to be the source of bleeding 21 (13.4%) of 157 unselected patients admitted to a general medical unit because of suspected upper gastrointestinal haemorrhage over a period of 30 months. They represented 16.7% of the 126 patients in whom there was ultimately good evidence of upper gastrointestinal blood-loss. In the total series of 200 suspected bleeding episodes the incidence was 11.5%. It is suggested that endoscopy of the upper gastrointestinal tract, peformed as early as possible after resuscitation, enables the diagnosis of the Mallory-Weiss lesion to be made, shows that it is a relatively common source of haemorrhage, and establishes whether bleeding has stopped, thereby helping to identify patients who can be managed without transfusion and those who are likely to require surgery.