Progressive diaphyseal dysplasia was found in a 3-generation family including 18 affected individuals. We describe the clinical and radiographic manifestations in 6 of 18 patients with this autosomal-dominant bone dysplasia and the good symptomatic response to corticosteroid treatment in one of these. The variability of manifestations of the disease in this family and in others previously described seems to depend on the sex of the patient and the parental origin of the mutation. The patients with more severe symptoms are males who inherited an allele of paternal origin. We suggest that the progressive diaphyseal dysplasia gene has a function in endochondral bone formation and that its mutation is a dynamic one with repeat expansion enhanced in father-to-son transmission.