Employing Northern blot analysis and the polymerase chain reaction, we investigated PRAD1 gene overexpression in the tumour tissues of 58 patients with B-cell lymphoma. These findings were then examined in relation to the patients' clinical and immunohistological characteristics. The over-expression of this gene was detected in 6/8 patients with mantle cell lymphoma (MCL) and in only 1/50 other lymphomas, indicating its close association with MCL. The patients with MCL had common clinical findings of advanced disease with generalized lymphadenopathy on admission, and they had a CD5+CD10-IgD+ phenotype. The patients with chronic lymphocytic leukaemia (CLL) also showed findings indicating a distinctive disease entity: a CD5+CD10-IgD+ phenotype and lack of PRAD1 over-expression. In contrast, most patients with diffuse low-grade lymphoma other than MCL and CLL had localized extranodal disease, expressed a CD5-CD10-IgD- phenotype, and lacked PRAD1 over-expression. These findings suggest that extranodal low-grade lymphomas differ from nodal MCL and are not part of the spectrum of CLL.