A patient with history of B cell lymphoma treated with rituximab-based chemotherapy relapsed with a blastic CD4+/CD56+ neoplasm that was negative for CD20, CD79a and CD3. The relapse morphology and immunophenotyping were unusual and plasmacytoid dendritic cell (PDC) tumor enters the differential diagnosis. However, expressions of Oct-2 and CD10 in the relapse tumor were both more compatible with B cell than PDC lineage. Molecular investigations showed clonal rearrangements for both immunoglobulin heavy chain (IgH) and T cell receptor (TCR) gamma chain gene by polymerase chain reaction (PCR). Furthermore, a clonal relationship with the original B cell lymphoma was demonstrated for all PCR products. Our case illustrated the potential pitfalls and ambiguity of lineage classification based on morphology and immunophenotyping alone, especially for rare and poorly defined entities.