SummaryHistological transformation (HT) into diffuse large B-cell lymphoma (DLBCL) was documented in 37 of the 281 (13%; 95% CI, 9-18) follicular lymphoma (FL) patients treated at our institute from 1979 to 2007. HT occurred at a median of 2·75 years from initial FL diagnosis and HT rate was 15% at 10 years and 26% at 14 years, with a plateau from that point onward. Patients with bulky or extranodal disease, or those diagnosed before 1990 had a significantly higher risk of HT. When initial treatment strategies were taken into account, a reduced HT risk was seen in the patients initially managed with a 'watch and wait' policy, while the risk appeared significantly increased in the small subset of 18 patients initially managed with rituximab plus chemotherapy (P = 0·0005). HT was associated with a significantly shorter cause-specific survival (P = 0·0002).