Juvenile Paget's disease (JPD) is a rare condition with an autosomal recessive mode of inheritance. Typically presenting in infancy or early childhood, the disorder is characterized by a generalized widening of the long bones and thickening of the skull combined with sustained elevation of serum alkaline phosphatase levels. The extremely rapid bone turnover results in osteopenia, fractures, and progressive skeletal deformity. In 2002, mutations in TNFRSF11B, the gene encoding osteoprotegerin, were described as underlying JPD. We evaluated a patient with JPD at the clinical, biochemical, radiological, and molecular level. Mutation analysis of TNFRSF11B revealed a homozygous insertion/deletion in exon 5, predicted to result in truncation of the protein at amino acid 325. The residual activity of the mutated protein product was investigated by Western blotting and ELISA upon transient overexpression. Absence of the C-terminal domain abolished homodimerization and was shown to lead to a decreased capacity of the mutant protein to bind its ligand RANKL. We conclude that truncation of the C-terminal part of osteoprotegerin negatively affects functional activity. As a consequence, osteoclast formation and function are up-regulated, causing the increased bone turnover seen in this patient.