One hundred non-operated hip joints in 50 adult patients with active rheumatoid polyarthritis were examined by ultrasonography as well as by clinical methods and X-ray. In 15 hip joints in 11 patients, ultrasonography indicated intra-articular effusion (a distance between the joint capsule and bone of more than 7 mm in 14 and a difference between the two sides of more than 1.5 mm for one hip). There were no subjective symptoms associated with five of these hip joints, the X-ray findings were normal for 11 hips, and careful clinical examination showed normal findings for one hip joint. The routine clinical examination used in the hospital had revealed pathological findings in only two of these hip joints. It is concluded that ultrasonography may reveal intra-articular effusion or synovitis in clinically and radiologically apparently normal hip joints of patients with active rheumatoid arthritis. The increase in pain and restricted motion was statistically significant in the hips for which ultrasonography indicated intra-articular effusion. Hips that appear pathological in ultrasonography are thus synovitis, and do not fall within the range of normal variation for healthy hip joints.