The sizes of follicle centre cells in 15 specimens of follicular (centroblastic-centrocytic) non-Hodgkin's lymphoma and 15 specimens of reactive follicular hyperplasia have been measured. The findings differ from previous studies, where nuclei of cells from follicle centres and interfollicular areas were measured and revealed no significant difference between follicular lymphoma and reactive follicular hyperplasia. In the current study, by measuring only follicle centre cell nuclei, it has been found that, in reactive follicular hyperplasia, the mean nuclear maximum diameter (Dmax) and area are significantly greater than in centroblastic-centrocytic follicular lymphoma. In addition, the standard deviation of these data is greater for reactive follicular hyperplasia than follicular lymphoma, implying greater "scatter" or heterogeneity of the nuclear sizes in the follicle centre cells of the former than the latter. Thus, the size of these cells appears to be of value as an histological discriminator between these benign and malignant conditions.