CD10 expression in various grades and interfollicular infiltrates of follicular lymphoma (FL) has not been well documented. Immunohistochemical staining for CD10 (clone 56C6) was performed on paraffin-embedded tissue from 26 cases of classic FL. Negative or weak expression of CD10 was more frequent in grade III (5/6 [83%]) than in grade I FLs (3/15 [20%]). CD10+ interfollicular infiltrates were present in 16 cases. Six (38%) of 16 cases showed that CD10 expression was strong or moderate in follicular areas but weak or negative in interfollicular infiltrates. Our results suggest that CD10 expression is frequently weak to negative in grade III and in interfollicular infiltrates of FLs. Therefore, lack of CD10 expression on small specimens, such as from needle core biopsy or fine-needle aspiration, does not preclude the possibility of a diagnosis of FL. Furthermore, lack of CD10 expression in diffuse large B-cell lymphoma does not exclude the possibility that the neoplastic lymphocytes are of follicle center cell origin.