The purpose of this study was to investigate the intra-articular vascularization of the synovium with power Doppler sonography and correlate these findings with histopathologic al findings of synovial vascularity. Before arthroplasty or arthroscopy with synovectomy the knee joints of 20 patients with osteoarthritis or rheumatoid arthritis were examined with power Doppler sonography. Vascularity of the synovial membrane was classified semi quantitatively. The degree of the synovial vascularity was graded quantitatively using histological method. The strong correlation was found between qualitative power Doppler sonography results with histological findings of vascularity (Spearman's correlation coefficient was 0.85, p=0.0005). Power Doppler sonography may be a valuable tool to detect synovial vascularity and to assist clinicians in distinguishing between inflammatory and non-inflammatory synovium.