1. The three age types of acute haematogenous osteomyelitis are conditioned in their respective clinical features by the differing nature of their vascular bone pattern.
2. In the infant the condition causes severe and often permanent epiphysial damage and joint infection, a large involucrum but only transient damage to the shaft and metaphysis.
3. In the child the condition is responsible for extensive cortical damage with involucrum formation, but, except for some stimulation of growth, permanent damage to the growth cartilage and to joints is exceptional. Chronicity of the disease is rare if treatment has been effective.
4. In the adult acute osteomyelitis of the long bones is rare. It causes very frequent joint infection; the cortex is absorbed instead of sequestrating. The whole of the bone is invaded and frequently leaves chronic infection in the bone marrow.
5. The vascular characteristics of the bones in each age group and their relation to the onset of infection are described.
6. Some general directives for management based on these facts are suggested.