Clinically, the patients are well up to about 3 years when motor weakness is noted. The children fatigue more easily than before. They have difficulties arising from a horizontal or sitting position and their gait becomes cautious and halting. A diagnosis of polymyositis or dermatomyositis is frequently considered. In our patients the electromyograms were normal as were the muscle enzymes (except in one child who had a transient mild elevation of the serum transaminases and creatinine creatine kinase). Muscle biopsies in two of our patients gave ambiguous results. Within one to two years joint stiffness appears affecting first the hips, then the other joints of the lower limbs. A diagnosis of rheumatoid arthritis is commonly made. Morning stiffness and decreased mobility of the cervical spine add to that suspicion. Examination shows restricted mobility of the large and small joints, muscle weakness, and swollen finger joints. In contrast to rheumatoid arthritis, the swelling is not caused by soft tissue involvement but by osseous distention of the ends of the phalanges. The joints are not tender, red, or warm. There is no periarticular edema. A normal sedimentation rate, negative C-reactive protein, negative rheumatoid factors, and a histologically normal synovia eliminate the diagnosis of rheumatoid arthritis. Radiographic examination shows signs of a bone dysplasia: the vertebral bodies are flattened with anterior ossification defects (Fig. 1); the acetabular region of the pelvis is abnormal (Fig. 2) and the ends of the proximal and middle phalanges are distended (Fig. 3). The articular space may be narrow but the destructive bone changes characteristic of rheumatoid arthritis are not present. When the dysplastic bone changes are detected the working diagnosis is commonly changed to “rheumatoid arthritis with Scheuermann disease.”