The features and outcome of 21 children (12 boys, nine girls) with idopathic juvenile osteoporosis (IJO) followed for up to 23 yr are described. The mean age of onset was 7 yr (range 1-13 yr) with no sex difference; the main presenting symptoms were long bone fractures, pain in the back and difficulty in walking. Typically, radiographs demonstrated compression of the vertebrae and metaphyses of the long bones; bone histology sometimes showed an excess of osteocytes associated with woven bone; routine biochemistry was normal for age; and with three exceptions no abnormalities of extracted dermal collagen or collagen synthesized by fibroblasts were detected. Where circulating vitamin D metabolites were measured they were within the normal range; and hip and spine bone mineral density (measured by DXA) was strikingly low. Five patients are still growing and two are currently untraceable. Of the remaining 14 who are now adults, 11 have substantially or completely recovered and three are disabled. Since spontaneous recovery occurs it remains impossible to assess the many forms of treatment given.