This retrospective study analyzes the results of radioimmunotherapy (RIT) with (90)Y-ibritumomab tiuxetan in 94 Japanese patients with relapsed or refractory low-grade B cell non-Hodgkin lymphoma at a single institution. All patients had previously been administered with 1-8 (median 1) regimens of rituximab alone or combined with other chemotherapeutic regimens at a mean age of 64 years. The overall response rate was 90 % and the complete response (CR) rate was 69 %. The median overall survival was not reached and progression-free survival (PFS) was 26 months, respectively, for the early phase 50 patients during a median follow-up period of 46.5 months. In this cohort, the PFS rates for the 50 early phase patients who had undergone ≤2 and ≥3 previous regimens, and for those who achieved CR compared with those who did not (partial response, PR; stable disease, SD; progressive disease, PD) were 38 and 11 months, respectively. Multivariate analysis showed that these two factors were statistically significant (p = 0.0011 and p <0.0001, respectively). The overall incidence of grade ≥3 non-hematological toxicity was 9 %. Two patients died of treatment-related deteriorating hepatitis C. A second malignancy developed in two patients at 10.5 and 3.5 months after treatment. We recommend administering (90)Y-ibritumomab tiuxetan as early in the disease course as possible, and at the latest as a third-line therapy to maximize the benefits of RIT, which should improve the quality of life for patients.