Criteria for the specific diagnoses of the several arthritides, on the basis of exfoliative cytology examination of fluids aspirated from diseased joints, were derived from examinations of 126 specimens of synovial fluid. They were screened cytologically and wholly on the basis of the cytological findings were classified into seven groups: rheumatoid arthritis, osteoarthritis, traumatic arthritis, gouty arthritis, Reiter's syndrome, pigmented villonodualr synovitis, and septic arthritis. The cytological features of each group appeared relatively constant and reproducible. Subsequent comparison of the clinical and cytological diagnoses showed excellent correlation.