The MIB-1 labeling index, which is based on Ki67 immunostaining, is widely used to evaluate the proliferation of tumor cells in lymphoma. However, its clinical significance has not been fully assessed. We retrospectively evaluated the prognostic impact of the MIB-1 labeling index at the time of diagnosis, in 98 patients with follicular lymphoma (FL) grade 1–3b who were treated uniformly with rituximab plus cyclophosphamide, doxorubicin, vincristine, and prednisolone (R-CHOP) therapy. The 5-year progression-free survival (PFS) for an MIB-1 labeling index of ≥10% (n = 60) and <10% (n = 38) was 35% and 61%, respectively (P = 0.015). The 5-year overall survival (OS) for an MIB-1 labeling index of ≥10% and <10% was 77% and 92%, respectively (P = 0.025). Pathological grading was not correlated with PFS or OS. In multivariate analysis, an MIB-1 labeling index of ≥10% was independently associated with poor PFS and OS. In conclusion, an MIB-1 labeling index of 10% is a useful cut-off level for predicting the prognosis of patients with FL.