1. Uptake of intravenously administered radioactive technetium (99mTc) was measured over the knee joints in normal human volunteers, in patients with osteoarthritis and in groups of synovectomized and unoperated patients with rheumatoid arthritis. The uptake was compared with clinical indices of inflammation (pain, tenderness swelling and stiffness), and the clearance rate of intra-articularly injected radioactive xenon (133Xe). The 99mTc uptakes were found to be unrelated to the isotope dose and the day-to-day reproducibility was acceptable.

2. The mean uptake of 99mTc was within normal limits in osteoarthritis. Both in synovectomized and in unoperated rheumatoid arthritis 99mTc uptake was significantly higher than in normal subjects.

3. Of the clinical indices studied significant correlation of 99mTc uptake was found with pain and swelling in all groups of patients studied.

4. Faster clearance of 133Xe in unoperated rheumatoid arthritis correlated well with the higher 99mTc uptakes.

5. The results confirm that 99mTc uptakes are raised in inflammatory arthritis but not in degenerative arthritis. The relation of 99mTc uptake to the clinical indices of inflammation and to the 133Xe clearance from the joint is discussed.

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