Physical examination is often insufficient in distinguishing between joint effusion and inflamed synovium in the knee joints of patients with rheumatoid arthritis. The authors prospectively evaluated the role of intravenously administered gadopentetate dimeglumine in distinguishing between these two conditions. Fourteen patients with classic rheumatoid arthritis were examined first by a rheumatologist and then by means of magnetic resonance (MR) imaging with T1- and T2-weighted sequences. T1-weighted images were also obtained following the intravenous administration of gadopentetate dimeglumine. T1-weighted images obtained prior to contrast material administration demonstrated an identical low-intensity signal from both effusion and inflamed synovium, and T2-weighted images demonstrated increased signal intensity in both cases. Intravenous administration of gadopentetate dimeglumine allowed distinction between effusion and abnormal synovium, with the effusion remaining of low signal intensity and the synovium demonstrating enhancement and increased signal intensity. The authors conclude that the use of gadopentetate allows distinction between synovial thickening and joint effusion in the knee, which may affect treatment decisions.