Gray scale ultrasound at a frequency of 5.0 MHz was used as part of a study to evaluate the results of 90Y injection as therapy for rheumatoid arthritis of the knee. Popliteal cysts, suprapatellar effusions and synovial thickening in the suprapatellar pouch were demonstrated and ultrasound studies were correlated with the clinical and arthrographic findings. Gray scale ultrasound can be a useful adjunct in the evaluation and follow-up of rheumatoid arthritis of the knee.