The clinical significance of synovial monocytosis (greater than 50 p. cent) of inflammatory fluids (greater than 1,000 elements per mm3) was studied retrospectively in 43 patients. An accurate diagnosis was made 35 times: rheumatoid polyarthritis (13 cases), lupus disease (1 case), Gougerot-Sjögren syndrome (1 case), isolated infantile monoarthritis (1 case), sarcoidosis (1 case), seronegative spondylo-arthropathy (3 cases), microcrystalline arthritis (5 cases) arthritis revealing a blood disease (2 cases), idiopathic "lymphocytic" arthritis (3 cases), eosinophilic pseudo-allergenic arthritis (2 cases), aseptic osteonecrosis of the medial condyle (1 case), femoro-patellar arthrosis (1 case). Among the 8 other patient for whom no diagnosis was made, 6 had a spontaneously resolving acute affection, evoking the possibility of a viral etiology.