Adult, de novo B-cell lymphomas meeting the WHO morphologic criteria for atypical Burkitt/Burkitt-like lymphoma cause diagnostic difficulty for pathologists because the genetic and clinical characteristics of this group of lymphomas have not been clearly defined. Thirty-one such lymphomas, designated as Burkitt-like lymphomas (BLL), were selected based on morphologic features and evaluated for immunophenotype, MYC and BCL2 status, and clinical features. Nine childhood Burkitt lymphomas (BL) and 87 adult, de novo diffuse large B-cell lymphomas (DLBL) were similarly evaluated for comparison. The BL group demonstrated uniform characteristics: all had Burkitt lymphoma morphology, an identical immunophenotype (positive for CD20, CD10, bcl-6, CD43, and p53; negative for CD138, CD23, bcl-2), high MIB-1 positivity, IGH/MYC translocation, no IGH/BCL2 translocation, and all patients were alive at the last follow-up. The BLL and DLBL groups were heterogeneous. Burkitt-like morphology alone correlated with decreased survival. IGH/MYC or IGL/MYC fusion was identified in 11 of 27 (41%) BLL and 4 of 76 (5%) DLBL and was associated with decreased survival in both groups. MIB-1 positivity did not correlate with morphology, MYC abnormalities, or survival. We propose that adult B-cell lymphomas with BLL morphology are a phenotypically and genetically heterogeneous group of aggressive lymphomas, biologically distinct from childhood BL. Until biologically accurate subgroups within this morphologically defined group are identified, it is appears that both recognition of BLL morphology and direct evaluation for the presence of MYC fusion to immunoglobulin genes are important for identification of adult patients with poorer prognosis than those with DLBL.