Of 550 patients admitted with acute upper gastrointestinal hemorrhage, 143 with peptic ulcers containing stigmata of recent hemorrhage accessible to endoscopic therapy were included in a randomized comparison of neodymium yttrium aluminum garnet laser, heater probe, and no endoscopic therapy. The rebleeding rate in laser-treated patients (20%) was significantly less than in controls (42%; p less than 0.05), but in heater probe-treated patients (28%) it was not significantly different from either of the other two groups. The mortality rate in the laser group (2%) was not significantly different from either the heater probe (10%) or the control (9%) group. This trial has confirmed the efficacy of the Nd YAG laser but not that of the heater probe in the prevention of rebleeding from recently bleeding peptic ulcers.