The clinical significance of cytoplasmic inclusions(CPI) in synovial fluid(SF) examination was evaluated. We examined SF specimens collected from major rheumatology clinics in the Philadelphia area during the period of January to December 1995. Among 759 patients in the initial study group, 419 cases with established diagnoses and full synovial analyses were included. Their diagnoses and SF analysis results including leukocyte counts, differential counts and wet preparations were collected and analysed. Ninety seven of the 419 SF specimens were found to have CPI. CPI were found in SF from almost all rheumatic diseases. They were most likely to be found in inflammatory arthropathy including rheumatoid arthritis(RA, 46%), juvenile rheumatoid arthritis(JRA, 78%) and psoriatic arthritis(55%). On the contrary, CPI were least common in crystal-induced arthropathy among the inflammatory arthropathy. CPI were found 8 out of 98 gout cases(8%) and 2 among 53 calcium pyrophosphate dihydrate(CPPD) deposition disease(4%). In noninflammatory arthropathy, CPI were found in only 6 cases(6%) out of the 103 osteoarthritis(OA). In RA cases with non-inflammatory SF, 4 of the 20 SF(20%) had CPI while only 6% of OA SF had CPI. OA SF with CPI were all noninflammatory SF. In summary, CPI were a common finding on SF examination. CPI were more likely to be found in inflammatory arthropathy than noninflammatory. Among inflammatory arthropathy, CPI can favor non-crystal arthropathy than crystal arthropathy. Awareness of the presence of CPI is suggested as an addendum to routine SF analysis. Renewed investigation of the several types of CPI may add further to the understanding of joint disease.