We have studied the expression of MIB-1 and prognosis in cyclin D1(CyD1)+ and CyD1- mantle cell lymphoma (MCL), and compared them to B-CLL/SLL. All cases were assigned to four groups by immunoreactivity and primary sites: (1) CyD1+ nodal MCL, 11 cases: (2) CyD1+ extranodal MCL (multiple lymphomatous polyposis, (MLP)) three cases: (3) CyD1- nodal MCL, three cases: and (4) CyD1- B-CLL/SLL, seven cases. The average of MIB-1 labeling indexes of the four groups were 30.66, 8.70, 9.30 and 4.66, respectively. The CyD1- group consisting of nodal MCL and CLL/SLL had a significantly longer median survival time (69 months) than the CyD1+ group consisting of nodal MCL and MLP (22 months, P = 0.01). These data indicate that CyD1- nodal MCL may show a lower MIB-1 labeling index, and has a better prognosis, than CyD1+ nodal MCL. In addition, a large difference in the average of MIB-1 labeling indexes between nodal MCL and MLP in the CyD1+ group was found.