Coinfection with HTLV-1 (T lymphotropic virus type I) has been observed in approximately 38% of cases of Strongyloides stercoralis infection in Japan. In the present study, we investigated whether the dose of ivermectin corresponding to approximately twice the conventional therapeutic dose could improve the anthelmintic rate without adverse effects in patients with intestinal strongyloidiasis, particularly in those positive for anti-HTLV-1 antibody. A single dose of 6 mg ivermectin (mean, 110 microg/kg) was administered and the same single dose was repeated 2 weeks later in 312 patients with intestinal strongyloidiasis during the period from 1990 to 1999. The long-term anthelmintic rate during the period of 12 months from 4 months after treatment was 77% (117/152) in all patients, and 92.7% (89/96) and 50% (28/56) in those negative and positive for anti-HTLV-1 antibody, respectively. Between 2000 and 2003, ivermectin was administered at 200 microg/kg in 97 patients, and repeated 2 weeks later. The long-term anthelmintic rate was 96.8% (60/62) in all patients, and 100% (42/42) and 90% (18/20) in those negative and positive for anti-HTLV-1 antibody, respectively. These results showed that the long-term anthelmintic rates in all patients in the 200 microg/kg dose group, and especially in those positive for anti-HTLV-1 antibody, were significantly higher than the respective rates of the 110 microg/kg dose group. Based on these results and the observed safety of the double dose of ivermectin, the recommended dose of ivermectin for treatment of intestinal strongyloidiasis should be 200 microg/kg in patients positive for anti-HTLV-1 antibody.