A 7-year-old female child presenting with hypophosphatemia associated with a severe degree of vitamin D-resistant rickets of the "simple" type is described. Genetic studies of 30 close relatives on both sides revealed no other persons with the disease; chemical studies of 13 relatives showed no instances of occult hypophosphatemia. No clinical or biochemical features other than the unusual severity of the bone disease were uncovered which unequivocally distinguish this patient from the more common sex-linked dominant form of the disease. There are a number of possible explanations for the occurrence of such "sporadic" cases of vitamin D-resistant rickets. The possibilities of either homozygosity for an unrecognized autosomal recessive form or of a new mutation are regarded as being more likely than phenocopy or acquired disease, but no one explanation can be accepted to the exclusion of the others at this point. It is emphasized that important results can be anticipated from a collation and study of a group of fully validated "sporadic" cases of the disease. The authors would like to propose one of us (J.B.G.) to serve as a clearing house for information regarding any similar "sporadic" cases known to other investigators in the hope that a suitably large group can be collected for analysis.