1. Aseptic necrosis of bone is a serious chronic complication of deep-sea diving and compressed-air work. 2. The changes to the bone which occur in this condition take time to develop to the stage where they cause the radiographic signs of bone necrosis, and consequently there is a delay of some months between the causal incident and the first diagnosis by radiography. 3. As a possible method for the earlier detection of bone necrosis the 24 h urinary excretion of hydroxyproline was measured over a period before and after experimental production of bone necrosis in rabbits by the intra-arterial injection of glass microspheres. 4. Total hydroxyproline excretion rose significantly within a few days of the injection in those rabbits in which there was later shown to be historical evidence of bone necrosis. This rise occurred long before there was any radiographic changes. 5. It is suggested that measurements of urinary hydroxyproline might be used to give an early indication of bone necrosis in man.