We describe a boy and his mother affected with craniodiaphyseal dysplasia (CDD). The boy had a very severe form of the disease with extensive osteosclerosis already at birth. Facial diplegia, bilateral hearing loss and optic nerve atrophy were early, severe complications of the disease. At age 7 years progressive genu valgum and unusual epimetaphyseal radiographic appearances suggested hyperparathyroidism. This was confirmed by biochemical tests. Because of some facial similarity between the asymptomatic mother and the propositus, a limited skeletal survey of the mother was performed. It demonstrated cranial osteosclerosis and hyperostosis. It is possible that the mother has somatic mosaicism for a mutation of the genes causing CDD.