Grade (G) 3B follicular lymphoma (FL) is a rare FL subtype which exists on a histological continuum between ‘low-grade’ (Grade 1, 2 and 3A FL) and diffuse large B-cell lymphoma (DLBCL) appearing to share features with each. Clinical characteristics and outcomes are poorly understood due to lack of adequate representation in prospective trials and large-scale analyses. We analyzed 157 G3BFL cases from 18 international centers, and two comparator groups; G3AFL (n=302) and DLBCL (n=548). Composite histology with DLBCL or low-grade FL occurred in approximately half of G3BFL cases. With median 5 years follow-up, G3BFL overall (OS; P.