1. Skeletal and other clinical features in twenty-three patients with homocystinuria have been compared with those in sixteen patients with Marfan's syndrome.

2. The two diseases are clinically similar but florid arachnodactyly and scoliosis are commoner in Marfan's syndrome, whereas widening of epiphyses and metaphyses of long bones is a distinctive feature of homocystinuria.

3. Patients with homocystinuria frequently have osteoporosis at a young age with a high incidence of vertebral involvement including biconcavity and flattening. Patients with Marfan's syndrome do not have osteoporosis and may have excessively tall vertebrae.

4. Mental retardation and thrombosis are common in homocystinuria and uncommon in Marfan's syndrome.

5. Homocystinuria is most probably inherited as an autosomal recessive and Marfan's syndrome as an autosomal dominant.

6. The two diseases should be differentiated because of the thrombotic risk in homocystinuria, and also because in this disease there is a possibility of treating the biochemical defect.

7. Although patients with homocystinuria may present to the orthopaedic surgeon with osteoporosis, severe genu valgum or scoliosis, the disease is an uncommon cause of these defects.