Cytomorphological categories of the Lukes and Collins classification could be recognized in 92% out of a retrospective series of 301 non-Hodgkin's lymphomas. The follicular center cell (FCC) group constituted the majority (54%). Certain differences between the cytological subgroups were observed as regards age and sex and the growth pattern. Follicular features in the growth pattern were found in 16% of the cases. The large cleaved FCC type behaved differently from the other FCC types in many respects. The occurrence of cleaved FCC lymphomas appeared to be exceptional in childhood. There were no small lymphocytic and plasmacytoid lymphocytic lymphomas in children or young adults, and large non-cleaved FCC lymphomas and immunoblastic sarcomas were rare in these. In the small non-cleaved FCC group both the Burkitt type and the non-Burkitt type exhibited a bimodal age distribution with a peak under 10 years of age and another at age 60-70 years. Five convoluted lymphocyte lymphomas, between 2 and 29 years of age, were found in this series. The heterogeneous group of unclassified lymphomas included as a special case one lymphoma with deeply multilobated nuclei.