Synovium was collected from 15 patients who were undergoing joint surgery. Two groups were defined by clinical diagnosis: patients with primary osteoarthritis (n = 4); and those with rheumatoid arthritis (n = 11). The synovium was studied using histological and morphometric techniques. In agreement with previous studies, no histological features specific for either diagnosis were found. A previously validated morphometric method was used to estimate the cellular density of randomly picked fields within defined areas of synovium. The mean nuclear density of cellularity of comparable areas of synovium was significantly different between these two disease states, but the mean nuclear density between individual representative samples within each clinical group was homogeneous. The morphometric analysis of lymphocyte subsets showed that within the upper synovial region and cellular aggregates in osteoarthritis, the distribution of T cells expressing the CD4 and CD8 antigen was the same. In rheumatoid arthritis CD8 cells predominated in the upper synovial region and CD4 cells in the cellular aggregates. Plasma cells were rarely found in osteoarthritic synovia, but were common in rheumatoid arthritis, with IgG-producing plasma cells predominating. Morphometric studies of representative synovial samples may help to improve histological diagnosis and our understanding of pathological mechanisms.