Transmission electron microscopy was used to study the ultrastructure of nucleoli in seven high- and seven low-grade non-Hodgkin's lymphomas. Nucleoli with a single large fibrillar centre were predominantly seen in the low-grade group, while the majority of nucleoli in high-grade lesions contained several small fibrillar centres of approximately equal size. A third type of nucleolar appearance was identified, with a large central fibrillar centre surrounded by numerous small 'satellite' fibrillar centres. This nucleolar configuration was the least common but was more often seen in low-grade than in high-grade lymphomas. The mean fibrillar centre cross-sectional area was determined for each group and measured 278,800 nm2 for low-grade non-Hodgkin's lymphomas and 62,507 nm2 for high-grade lymphomas (P less than 0.001). These results show that fibrillar centre size and morphology correlates with grade of non-Hodgkin's lymphoma and may reflect differences in ribosomal deoxyribonucleic acid transcription.