In this comparative analysis of laboratory data, we examined the characteristics of synovial fluid leukocytosis in eighty adult patients with bacterial arthritis, reactive arthritis or rheumatoid arthritis of the knee joint. Synovial fluid leukocyte count and the percentage of polymorphonuclear cells seemed to perform well as a discriminator between bacterial infection and acute flare of the underlying disease in patients with rheumatoid arthritis. In contrast, there were no definite difference in the intensity of synovial fluid leukocytosis between patients with bacterial arthritis caused by living bacteria and patients with reactive arthritis probably caused by bacterial antigens.