Magnetic resonance imaging (MRI) of 18 knees of patients with arthritis was performed before and immediately after arthrocentesis. Pre- and post-aspiration 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. The difference between MRI-determined and syringe-determined volumes of aspirated joint fluid was 0-7 ml, median 2 ml, corresponding to 0-18%, median 7%, of the pre-aspiration effusion volume. Synovial membrane volumes, determined before and after arthrocentesis varied 0-10 ml, median 3 ml (0-17%, median 7%). No significant systematic misinterpretation of the borderline between joint fluid and synovium was found. We conclude that effusion volumes and in all probability also synovial membrane volumes, can be determined by MRI with a maximal analytical error of approximately 20%. The acceptable accuracy of the method encourages further studies of the value of effusion and synovial membrane volumes as markers of the activity and/or severity of joint inflammation.