The clinical presentation of 41 adult patients with infectious arthritis has been reviewed with special emphasis on initial synovial fluid leukocytosis. Fifty percent of the patients with culture-proven joint-space infections had synovial fluid leukocyte counts below 28,000/mm3. Comparison of this investigation with previous studies of similar magnitude demonstrates a striking difference in the mean and median synovial fluid white cell counts. The population reviewed had a higher incidence of patients with potentially immunocompromising medical conditions than previous reports. Similarities between this and previous reports include predisposing conditions, the spectrum of pathogens cultured, associated clinical findings on admission, hospital course, and mortality. The data presented here document the magnitude of potential overlap between the synovial fluid leukocytosis in infected joints and in joints afflicted with other forms of inflammatory arthropathy. Three patients populations (malignant neoplasms, steroid use, and intravenous drug abuse) with positive cultures from synovial fluid aspirates but initial synovial fluid white cell counts averaging below 50,000 cells/mm3 were identified. Patients with moderate synovial fluid leukocytosis, especially those potentially immunocompromised, must be considered to have infectious arthritis unless other causes of inflammatory arthropathies are demonstrated.