The objective of this study was to verify the accuracy of ultrasonography in assessing the topography, morphology, and extent of synovial proliferation in rheumatoid and psoriatic knee joint synovitis. Findings were compared to those obtained using prospective arthroscopy as the gold standard; in addition, topographically defined sonographic findings before and after arthroscopic synovectomy were compared. Sonographic examination was performed in 12 patients with rheumatoid arthritis (13 knees) and 13 patients with psoriatic arthritis (14 knees) who had synovitis of the knee using an electronic linear transducer (7.5 MHz) or a mechanical sector transducer (10 MHz). This examination was followed within 1 week by arthroscopy, to compare the topography (intra-articular localization) and the morphology (sonographic patterns) of synovial proliferation. In 15 knees undergoing arthroscopic synovectomy, preoperative sonographic measurement of synovial thickness in the suprapatellar, medial parapatellar, and lateral parapatellar recesses was compared with arthroscopic visualization of synovial proliferation; 13 knees were reevaluated 2 months after arthroscopic synovectomy by sonography at the same sites. Three distinct sonographic patterns of synovial proliferation were confirmed by arthroscopic examination: a villonodular aspect in 12 knees; uniform thickening in eight knees, and overlapping layers in seven knees. About 50% of the knees showed more than one sonographic pattern, with no differences in pattern distribution between rheumatoid arthritis and psoriatic arthritis patients. A significant correlation was found between sonographic and arthroscopic evaluations of synovial thickness in the suprapatellar (P < 0.02) and medial parapateoffr recesses (P < 0.02), the sites of maximal synovial proliferation in our patients.(ABSTRACT TRUNCATED AT 250 WORDS)