A 62-year-old previously healthy woman presented with a centroblastic non-Hodgkin's lymphoma in the thyroid. Chromosome analysis revealed two unrelated clones, 47,XX,+5 and 46,XX,-14,+der(14)t(5;14)(q11;q32). The two clones may reflect a polyclonal origin, or they may be the descendants of the same neoplastically rearranged cell. In the latter case, the clonal aberrations are either secondary to an event detectable only at the molecular level, or one of them is a primary cytogenetic event while the other arose through clonal evolution with loss of the primary aberration. The best candidate for the primary change would be trisomy 5. Trisomy 5 has previously been associated with lymphomas with diffuse, large, noncleaved morphology, a group within the Working Formulation largely equivalent to centroblastic lymphomas in the Kiel classification. Our findings thus support the notion that trisomy 5 may be associated with centroblastic/diffuse, large, noncleaved lymphomas.