Fifty-two B cell non-Hodgkin's lymphomas, in which the diagnosis was based on immunological, cytochemical, and ultrastructural studies, were characterised by morphometry on plastic-embedded tissue sections. Parameters studied were: nuclear size, cytoplasmic area, cytoplasm to nucleus ratio, nuclear contour index, nucleolar size, location of nucleoli within the nucleus, as expressed by relative nucleolar eccentricity, and the mean number of nucleoli per nuclear cross-section. The results of the measurements and subsequent statistical analysis show that the different types of lymphoma can be distinguished from each other, even though the differences were small. Small cell lymphomas (chronic lymphocytic leukaemia, lymphoplasmacytoid and polymorphic immunocytoma, centrocytic, centroblastic/centrocytic and intermediate lymphocytic lymphomas) could be separated from each other by the standard error of mean (SEM) of nuclear area, the cytoplasm to nucleus ratio, and nuclear contour index. Large cell lymphomas (centroblastic and B immunoblastic lymphomas) could be differentiated with cytoplasm to nucleus ratio and nucleolar parameters (relative nucleolar eccentricity and number of nucleoli per nuclear cross-section). The morphometric parameters of lymphoblastic lymphomas fell in the range of the large cell lymphomas, but lower SEM indicate these lymphomas were more monomorphic. These morphometric data underline the concept of the Kiel classification and establish the usefulness of morphometry as an additional technique in diagnosis.