The efficacy of mebendazole against enterobiasis was tested on 150 institutionalized mental retardates (range 5-25 years). At the start of the trial all patients swallowed on tablet containing 100 mg mebendazole. Another tablet was administered to all patients 2 weeks and 1, 2 and 3 months later. 19 patients were further treated with one tablet 4 and 5 months after the start of the trial. The presence of E. vermicularis was checked with cellophane adhesive tapes from the anal skin and from the right-hand finger-tips before the trial and after 1, 3 and 6 months. 94 subjects were positive before the trial. At the first post-treatment examination, all patients were found negative. However, 12% were reinfected. No side effects were observed or reported.