1. A child is described who presented with very severe rickets and gross myopathy. The clinical, biochemical and radiological signs were identical with those to be expected of a very chronic and severe vitamin D deficiency. The child's diet, however, had been normal.

2. All the pathological signs, except for residual dwarfism and leg bowing, disappeared on treatment with very large doses of vitamin D₂. Ordinary anti-rachitic doses had no effect.

3. We suggest that this child demonstrates a true resistance to the action of vitamin D and that the defect is permanent. The findings in two similar patients that we have seen suggest that the condition is inherited as an autosomal recessive gene, and that it may be the same disease as that described in the continental literature as "hereditäre pseudo-mangelrachitis" and by other names.

4. The disease seems distinct clinically and biochemically from the disease originally described under the name "vitamin resistant rickets," which does not respond so well to massive vitamin D therapy and which is usually inherited as a sex-linked dominant gene.

1. A clinical, radiological and histological description of a patient with fibrogenesis imperfecta ossium is given. We think that this is the first case in which diagnosis has been made during the life of the patient.

2. The disease is characterised by a defect in the formation of the collagen fibres of the bone matrix. There is also a failure of normal calcification of the matrix, giving rise to the appearance of wide "osteoid" seams. When examined with the polarising microscope and when stained with Gomori's reticulin stain the collagen fibres can be seen to be grossly deficient and abnormal.

3. The patient presented at the age of fifty-four years with bone pain and multiple fractures. The only biochemical abnormality detected in the plasma was an elevated alkaline phosphatase. He was also in negative calcium balance.

4. Treatment with vitamin D₂, later changed to dihydrotachysterol, appears to have produced clinical, biochemical and radiological improvement. It appears that a direct action of the vitamin on the abnormal bone collagen must be postulated, in addition to its known actions on the calcifying mechanisms.

5. An unusual feature of the case was the slow development of a total unresponsiveness to large doses of vitamin D₂, in spite of a markedly elevated level of vitamin D in the plasma. There was later a response to a much smaller dose of dihydrotachysterol, which is being maintained to date.