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 one 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.