Five hundred and nineteen patients with follicular center cell (FCC) tumors in Osaka, Japan were selected for study from 1000 patients with non-Hodgkin's lymphoma (NHL). They were admitted to hospital during 1964-1987, which was divided into three periods 1964-1972 (I), 1973-1979 (II), and 1980-1987 (III) to examine the time trend of FCC tumors. The median age of patients rose from 51 to 55 years of age with an increase in female and extranodal disease patients. The frequency of nodal FCC tumors among all nodal NHL increased from 37.5 to 51.5% (P less than 0.05), but that of extranodal types decreased from 82.9 to 56.8% (P less than 0.01), although the total number showed little change. When the cases were categorized as diffuse type and follicular type, the diffuse type predominated through the period in both nodal and extranodal sites. The frequencies of follicular type among all NHL were 10.2, 6.3, and 8.4% during the periods I, II and III, respectively. These results showed that follicular lymphoma was a minor constituent of NHL in Japan compared to Western countries, and the frequency did not increase from 1964 to 1987. The frequency of FCC tumors in the present series was rather higher than that in Western countries, therefore it is concluded that FCC tumors in Japanese have much less potential for forming a follicular structure than those in patients from Western countries.