Wear particles from prosthetic implants have been shown to cause inflammatory synovitis and periprosthetic osteolysis. These particle-induced pathologies are manifestations of adverse cellular responses to phagocytosed particles. In this study, phagocytosis of polyethylene particles was analyzed using flow cytometry (FCM), and the clinical utility of FCM in diagnosing particle-induced synovitis was examined. Ultra high molecular weight polyethylene particles exhibited natural autofluorescence at fluorescein isothiocyanate wavelengths when determined by FCM. Using this autofluorescent property of the particles, peripheral blood monocytes (PBMs) phagocytosing the particles could be detected by autofluorescence emission from intracellular particles. This autofluorescence from PBMs increased with particles/cell ratio in a dose-dependent manner. Particle phagocytosis was also detectable in joint fluid cells obtained from the patients with particle-induced synovitis following total joint arthroplasty (TJA). Phenotypic analysis indicated that phagocytes were typically CD14(+)CD16(-) macrophages, with occasional CD14(+)CD16(+) macrophages. Interestingly, decreased autofluorescence intensity of CD14(+) cells was observed after arthroscopic drainage, suggesting that FCM was useful in examining whether the treatment was successful. In summary, these results indicate that FCM analysis offers a simple and useful method of detecting phagocytosis of polyethylene particles and estimating the severity of particle-induced synovitis in post-TJA patients.