The clinical, pathologic, and genetic findings of a boy with intranuclear nemaline rod myopathy are described. Serial muscle biopsies revealed myocyte nuclei containing inclusions that were immunoreactive for alpha-actinin and increased with age. Genetic analysis revealed a Val163Leu ACTA1 mutation previously associated with nemaline rod myopathy. Although initially delayed, he has reached all milestones and remains stable. These findings suggest intranuclear rods may increase with time and do not necessarily imply a poor prognosis.