Two siblings, a 3-month-old white male infant and a 12-day-old female infant, had an anterior mandibular bony lesion that, in both cases, had been present at birth. After evaluation of clinical, physical, radiographic, laboratory, and histologic findings, a diagnosis of congenital monostotic fibrous dysplasia was made. Thorough review of the literature on fibrous dysplasia yielded no similar cases. The two cases presented appear to be the first reported examples of congenital monostotic fibrous dysplasia in siblings. The parents said there was no consanguinity. The possibility of a new autosomal-recessive disorder is likely.