We compared power Doppler sonography to laboratory indices of disease activity in patients with knee arthritis to determine the clinical relevance of hypervascularity. Eight healthy volunteers and 22 patients with symptoms and signs of knee arthritis were studied. Presence or absence of hypervascularity, synovial thickening, effusion, and Baker's cysts were recorded. Disease activity was measured by erythrocyte sedimentation rate, c-reactive protein, alpha2-globulins, sideremia, hemoglobinemia, and serum white cell count. Various grades of synovial hyperemia were found in 12/22 cases. Patients with and without synovial hypervascularity showed statistically significant differences in age (P=0.017), erythrocyte sedimentation rate (ESR) (P = 0.039), hemoglobinemia (P = 0.009), and sideremia (P = 0.012). Power Doppler sonography is able to demonstrate synovial hyperemia, which is correlated with some laboratory indices of inflammation.