The clinical use of nonsteroidal anti-inflammatory drugs (NSAIDs) is associated with significant adverse effects on the integrity of the gastrointestinal (GI) mucosa. A unique, double-blind, placebo-controlled, randomized, multicentre study investigated the prophylactic co-therapy with misoprostol, a novel PGE1 analog, for the prevention of the NSAID-induced gastric and duodenal mucosal lesions. The study also investigated whether the co-therapy with misoprostol could interfere with the anti-rheumatic action of the NSAIDs using detailed rheumatological assessments. Patients with osteoarthritis or rheumatoid arthritis had to be free of symptoms and significant erosive and/or haemorrhagic lesions of the upper GI tract. The patients were randomized to co-therapy with misoprostol or its matching placebo. Follow-up endoscopy and symptoms assessment were carried out within 4 weeks and compared to pre-study findings. Misoprostol significantly reduced (p less than 0.01) the incidence of erosive and/or haemorrhagic gastric and duodenal mucosal lesions. Misoprostol also reduced the proportion of patients with epigastric pain (p less than 0.01). Misoprostol was well tolerated and did not interfere with the anti-rheumatic activity of the administered NSAID. We conclude that misoprostol is safe and effective in the protection against NSAID-induced gastric and duodenal mucosal lesions and symptoms.