A diffuse large-cell B lymphoma (DLBL) is an aggressively growing lymphoma with, as the name suggests, the presence of greatly enlarged lymphocytes with an abnormally large nucleus. They often have an irregular cytoplasm and the nucleus usually exhibits one very prominent nucleolus.

Diagnosis relies mainly on the histology of a lymph node biopsy. Flow cytometry is hardly of added value, except for the demonstration of a monoclonal B-cell population (in this case 40%).