Cytologic study of synovial fluid has been relatively neglected both by cytologists and rheumatologists. Currently identified implications of cytologic findings include definitive diagnoses of a variety of joint diseases such as those due to malignancies, infections, or pathogenic crystals. Diagnoses can also be strongly suggested, for example, by visualization of lupus erythematosus cells formed in vivo. Immunocytochemistry is being explored for diagnostic and pathogenetic implications. We propose that complementary techniques used in our cytology and rheumatology laboratories may provide new insights into the clinical significance of cellular changes in joint fluids.