The prognostic value of grading follicular lymphoma has been debated since the 1980s. There is consensus that World Health Organization (WHO) grades 1 and 2 are indolent, but not whether grades 3A or 3B are aggressive. We retrospectively reviewed the follicular lymphoma diagnoses according to the 2008 WHO classification in all diagnostic specimens from a population-based cohort of 505 patients with a median follow-up time of 10·0 years (range, 4·6–16·0). After excluding 43 patients with concomitant diffuse large B-cell lymphoma, 345 remained with grade 1–2, 94 with grade 3A, and 23 with grade 3B follicular lymphoma. Grades 1–2 and 3A seemed equally indolent, with indistinguishable clinical courses, even in patients receiving anthracyclines. Compared with grades 1–3A and independently of clinical factors, grade 3B correlated with higher mortality (P =0·008), but outcome was improved after upfront anthracycline-containing therapy (P =0·015). In contrast to grade 1–3A patients, grade 3B patients experienced no relapses or deaths beyond 5 years of follow-up. Furthermore, patients with grade 3B were predominantly male and seldom presented with bone-marrow involvement. We conclude that follicular lymphoma grade 1–3A is indolent and incurable with conventional therapy. Grade 3B appears to be an aggressive but curable disease.