BCL1/PRAD1 is the gene locus involved in the t(11;14)(q13;q32) translocation, which often occurs in a proposed subtype of non-Hodgkin's lymphoma of B-cell phenotype (B-NHL), named mantle cell lymphoma (MCL). When 67 Japanese patients with B-NHL were examined using two separate probes composed of the BCL1 MTC probe and the PRADI cDNA probe, rearrangement of BCL1/PRAD1 or overexpression of PRAD1 was detected in 11 patients. Among 13 patients with MCL, 8 had the abnormalities (61%) and the MTC probe detected the BCL1 rearrangement in 5 (38%). Five of the 6 MCL patients studied (83%) showed PRAD1 overexpression. These frequencies were compatible with those reported for Western patients. Although the remaining three with BCL1/PRAD1 abnormalities were diagnosed as having other histologies, 11 patients had advanced diseases, with dissemination to the extranodal sites. Except for one with diffuse large cell lymphoma, they had a slowly progressive disease, and none of the patients displayed clinical or pathological transformation. The tumor cells usually expressed CD5 and lacked CD10. The cells were completely uniform in the expression of IgM and/or IgD, and in the absence of C mu gene deletion. It thus appears that B-malignancies involving the BCL1/PRAD1 locus constitute a refined disease entity.