Juvenile hyaline fibromatosis (JHF) is a rare systemic disease characterized by papulonodular skin lesions, gingival hyperplasia, joint contractures, and osteolytic lesions on long bones and the skull. It has recently been reported that the disease is caused by mutations in the gene encoding capillary morphogenesis protein-2 (CMG-2). To date, fewer than 60 cases have been published in the literature. Partial disease expression is common, but no cases featuring a solitary calvarial lesion have been reported. The authors discuss this 4-year-old boy with a solitary calvarial osteolytic lesion whose histopathological examination exhibited findings characteristic of JHF. Mutational analysis, however, revealed that there were no mutations in the CMG-2 gene. Two years after surgery, he was free of any complaints as well as gingival hyperplasia, joint contractures, and new skull or skin lesions. This patient's condition may represent clinical or genetic heterogeneity associated with JHF. Whether solitary lesions mimicking JHF can arise from somatic mutation of the CMG-2 gene remains to be proven.