We previously treated 47 patients with 100 mg of mebendazole (MBZ) twice a day by oral use for 5 days and this treatment was repeated 1, 3 and 4 weeks later. Although the cure rate was 100%, liver injury was observed in 48.9% of the patients. On this study, we reduced the periods of administration of MBZ (powder; 100 mg twice a day) to 4 days, and repeated it once after 3 days interval, and this initial treatment was performed one more time after 10 days interval (group 1). As Strongyloides stercoralis is mainly located in upper digestive systems, we used the drug reduced to powder for the purpose of better contact with the parasites. We considered that the powder should be absorbed well and liver injury occurred in high incidence. As group 2, we used the tablet itself in the same schedules of group 1. The results obtained were as follows; 1) The eradication rates at 10 days after the initial treatment were 97.8% (44 of 45 patients) in group 1 (powder) and 93.0% (40/43) in group 2 (tablet). 2) At 3 days after the whole treatment, the eradication rates were 100.0% in group 1, and 97.7% (42/43) in group 2. 3) Slight side effects such as constipation (6.7% in the group 1), dizziness or vertigo (6.7% in the group 1) and itching (6.7% in the group 2) were observed. 4) Liver injury was observed at 11.1% (5/45) 10 days after the initial treatment in the group 1 and 13.3% (6/45) in the group 2.(ABSTRACT TRUNCATED AT 250 WORDS)