Arthrocentesis has to be considered as a part of the clinical examination. A reasonable amount of aspirated synovial fluid is the best argument in favour of an objective articular disorder. Moreover some very simple evaluations are very helpful to make a diagnosis and to distinguish some particularities of rheumatic diseases. Such evaluations have to include both bacterial and synovial fluid analysis. Moreover, when performing synovial fluid analysis, a search for microcrystals is also performed. Haemarthrosis can easily be distinguished from a traumatic tap if the investigator is observing carefully the synovial fluid entering in the syringe. The diseases responsible for haemarthrosis differ with the age of patients: chondrocalcinosis, together with osteoarthrosis, is the most frequent aetiology in the elderly; disorders of haemostasis and synovial tumours are mostly observed in children and young adults. Paucicellular (< 1000 cells/mm3) synovial fluid is observed in different 'mechanical' disorders. In the case of purulent synovial fluid the primary diagnosis is septic arthritis. However, the most common aetiology is probably crystal-induced acute arthritis. Differential cell count analysis performed in case of 'inflammatory' (> 1000 or 2000 cells/mm3) synovial fluid usually shows a predominance of polymorphonuclear cells. However, high cellularity may sometimes be associated with a predominance of other cells, i.e. lymphocytes, monocytes, eosinophils. In this situation, such a simple evaluation (differential cell count analysis) is very helpful in making a diagnosis, e.g. eosinophilic arthritis, or to distinguish some particularities of rheumatic diseases, e.g. absence of cartilage breakdown in case of lymphocytic arthritis.