Volumes of synovial membrane and joint effusion were determined by magnetic resonance imaging (MRI) in patients with inflammatory gonarthritis. Volumes were calculated by adding the outlined areas of synovium/effusion from a continuous series of gadolinium-DTPA-enhanced 5 mm transversal T1-weighted MR-images. Ten knees with clinically active gonarthritis (CAG), 8 knees with clinically inactive gonarthritis (CIG) and 5 healthy controls (HC) were examined. The synovial volume of CAG-, CIG- and HC-knees were significantly different. The median volumes were 79 ml, 21 ml and 3 ml, respectively. Synovial and effusion volumes were highly correlated. A selected sagittal one-slice volume showed a high correlation to the total volumes. The reliable quantification of synovial and effusion volumes could be useful in the assessment of disease severity and as an outcome measure to treatment.