The anthracycline drug pirarubicin (tetrahydropyranyl adriamycin; THP) apparently has fewer cardiotoxic effects than doxorubicin. We previously described the benefit of the THP-COP regimen comprising cyclophosphamide, THP, vincristine, and prednisolone for elderly patients with diffuse large B-cell lymphoma (DLBCL). However, that study was completed before rituximab (R) was introduced into clinical practice. Here we report a phase II study of the THP-COP regimen combined with R (R-THP-COP) every 3 weeks. The complete response and 3-year overall survival rates was 63% and 53%, respectively, and no deaths were related to the regimen. We conclude that the R-THP-COP regimen is safe and effective for patients with DLBCL. Based on these results, a randomized controlled trial of rituximab-cyclophosphamide, doxorubicin, vincristine, and prednisolone (R-CHOP) and R-THP-COP as a phase III study is ongoing.