Misoprostol (Mi, a double blind study in the treatment of active duodenal ulcer (DU). The purpose of the paper was to evaluate the effect of Mi vs. Placebo (Pl) without antacids, assessing at the same time side effects and histological alterations. Thirty five patients with active DU diagnosed by endoscopy and photography (performed on days, 0, 14 and 28) were studied. They received 4 doses/day of Mi 100 mcg, or Pl double blind. In each control, biochemical studies in blood and urine were performed. Acetaminophen, 500 mg was the only analgesic allowed. Six patients were excluded. Of the remaining 29 patients, 15 received Mi and 14 Pl. There was no difference in healing at the 14 day stage. At the 28 day stage there was complete healing of the ulcer in 66.6%, and reduction of the ulcer diameter of 89% those who received Mi. The healing rate was 28.5% and reduction of the ulcer diameter of 31% on those patients who received placebo. There were no biochemical alterations or side effects. There was no significative difference between biopsies taken from pylorus and antrum. The consumption of acetaminophen was similar to both groups. It is concluded that Mi has a healing effect similar to H2 blocking agents, accelerating the healing of active DU, without side effects and biochemical changes.