OBJECTIVE To describe the presentation and clinical course of bicipital synovial cysts in 6 patients with juvenile rheumatoid arthritis (JRA) and to elucidate their anatomy using ultrasonography. METHODS A clinical description of the cyst, JRA subtype, activity of concurrent arthritis, systemic disease features, and erythrocyte sedimentation rate was recorded for each patient. Ultrasonographic examination of the cyst was performed for each patient. RESULTS Bicipital cysts generally presented as a sudden painless swelling on the flexor aspect of the upper arm, but on was in the forearm. Five of 6 patients had systemic disease. All patients had active arthritis when the cysts developed. Five of 6 cysts resolved spontaneously in less than 12 weeks. Ultrasonography confirmed a cystic structure in all patients, and in all cases was associated with thickening of the biceps tendon. Fluid was found within or adjacent to the biceps tendon in all cases. CONCLUSIONS Bicipital synovial cysts should be considered a cause of acute arm swelling in patients with JRA, especially those with systemic disease. Ultrasonography is a useful diagnostic adjunct in this condition. This condition is generally self-resolving.