Nucleolar organizer regions may be useful in the diagnosis and classification of non-Hodgkin's lymphomas. In this study of 46 cases we applied morphometric analysis with quantitation of physical descriptors of the nuclear profile (area, perimeter) and both number and area of stained nucleolar organizers therein enclosed to a series of lymphomas and benign lymphoid infiltrates. While nuclear outlines were manually traced small organizer regions within the nuclear profiles were semi-automatically outlined by a thresholding procedure subjected to manual override. This results in determination of number, area and perimeter of organizer regions. Data were corrected for section thickness effects and a stereologic (three-dimensional) analysis was additionally performed. We found an increase in mean number and area of nucleolar organizers per nucleus in high grade lymphomas compared to benign infiltrates and lower grade lymphomas. Volume and thickness corrected data showed a decrease in organizer number with concomitant increase in organizer volume in the higher grade lymphomas. Multivariate analysis of the cases, previously classified histologically, showed that clear resolution could be obtained, on the basis of physical descriptors, both between as well as within groups of the three tumor grades.