We investigated the existence of nanosize particles in synovial fluids of rheumatoid arthritis and osteoarthritis patients. These specimens were cultured under mammalian cell culture conditions (37 degrees C; 5% CO2/95% air) for a long period. After about 2 months, many nanoparticles appeared and they gradually increased in number and in size. The nanobacteria-like particles exist in synovial fluids of arthritis patients. The possibility of their existence and pathogenesis in various diseases should be verified cautiously.