Pediatric follicular lymphoma (PFL) is a variant of follicular lymphoma (FL) presenting as localized lymphadenopathy in children. Unlike conventional adult FL, PFL typically does not recur or progress. Clear diagnostic criteria for PFL are lacking, and it is uncertain whether this indolent lymphoma is defined by age or may occur in adults. We analyzed 27 FL in patients < 40 years of age and found that all 21 cases that lacked a BCL2 gene abnormality (BCL2-N; P < .0001) and had > 30% Ki67 fraction (high proliferation index, HPI; P = .0007) were stage I and did not progress or recur; in comparison, all 6 cases with BCL2 rearrangement and/or PI < 30% were stage III/IV, and 5 of 6 recurred or progressed. In a separate cohort of 58 adult FL (≥ 18 years of age), all 13 BCL2-N/HPI cases were stage I, and none progressed or relapsed, whereas 11 of 15 stage I cases with BCL2 gene abnormality and/or LPI relapsed or progressed (P = .0001). The adult and pediatric BCL2-N/HPI FL cases had similar morphologic features. Our results confirm the highly indolent behavior of PFL and suggest that these are characterized by HPI and absence of BCL2 gene abnormality. PFL-like cases also occur in adults and are associated with indolent behavior in this patient population.