Infantile cortical hyperostosis (ICH) is characterized by spontaneous episodes of subperiosteal new bone formation in the long bones, mandible, and clavicle during infancy. A heterozygous missense mutation, c.3040C > T (p.R1014C), in the type I collagen alpha1 chain gene (COL1A1) was reported in families with the autosomal dominant form of ICH. We examined six consecutive cases of ICH from five unrelated families and their parents. The mutation was identified in all patients and two parents tested. Our result supported that ICH is caused by the single mutation in COL1A1 with incomplete penetrance.