Ultrasonography (US) of the knee in gonarthritis was evaluated with gadolinium-DTPA-enhanced MR imaging as a reference. The study included 13 patients with gonarthritis, 2 patients with osteoarthritis and 5 healthy controls. Compared to MR, US identified 100% (12/12) of joint effusions, 100% (5/5) of Baker's cysts, 57% (8/14) of the synovial membranes of the suprapatellar recess and 38% (3/8) of bone erosions. No MR or clinical findings characterized the patients with ultrasonographically identifiable synovial membranes. Quantitative MR- and US-estimates of effusion, synovial membrane and cartilage thicknesses were correlated. The Spearman correlation coefficients were 0.87, 0.86 and 0.82, respectively. US reliably demonstrated joint effusions and Baker's cysts but showed lack of sensitivity for demonstration of the synovial membrane and bone erosions. When the synovial membrane was identifiable, the measurements of synovial thickness by US appeared precise. Cartilage evaluation by US seemed dependable. The clinical value was limited, because the weight-bearing areas were inaccessible.