1. The syndrome of osteoporosis is reviewed and its various causes are mentioned. Osteoporosis in youngish patients without any demonstrable cause is referred to as "idiopathic." The scant literature on this condition is reviewed. Its clinical, radiological, biochemical and histological features are considered.

2. A series of thirty-eight cases is analysed, and illustrative case histories are described. The peculiarities of the disease as it is seen in women are discussed, particularly the relationship to pregnancy and lactation, which appear to act as precipitating factors, rather than being primarily causative.

3. The differential diagnosis is discussed. Osteogenesis imperfecta may not always be easy to distinguish; since it is really a "congenital osteoporosis" this is hardly surprising.

4. The following possible etiological factors are propounded (apart from pregnancy): nutritional, occupational, lack of sex hormone, liver dysfunction, loss of protein, diabetes, premature ageing, hypophosphatasia, "alarm reaction," and inheritance. None of them can be incriminated except in the odd case. The relationship between osteoporosis and idiopathic hypercalcuria is mentioned. The only conclusion regarding etiology is that some people are simply more prone to bone loss than are others, and in these a variety of accentuating factors may render the disorder clinically apparent.

5. The treatment of the condition is unsatisfactory, although occasionally a positive calcium balance may be obtained with sex hormones or intravenous infusion of plasma albumin or whole plasma. The general tendency seems to be towards clinical improvement (biologically "stabilisation" rather than improvement), but some patients become permanently crippled.