The case histories of five black children aged between 4 1/2 and 9 years, with acquired hypophosphataemic vitamin D resistant rickets are presented. Muscle weakness was a striking feature clinically, while radiographically all had marked osteopenia in association with classical florid rickets. Biochemically, they were normocalcaemic and hypophosphataemic, with normal parathyroid hormone concentrations. In three of the children 1,25-dihydroxyvitamin D concentrations were low. One child responded to no specific therapy, while three required continuous 1 alpha-hydroxyvitamin D and oral phosphate supplementation, and the fifth 1 alpha-hydroxyvitamin D therapy alone before the clinical, biochemical and radiographic picture responded. The features in three of the children resemble oncogenous rickets, but the features of the other two do not fit any known aetiology. Although the group as a whole may be heterogeneous, until the pathogenetic mechanisms are determined, the authors believe they should be grouped together as acquired hypophosphataemic vitamin D resistant rickets.