Two siblings who developed adult form acid maltase deficiency (AMD) are reported. The elder sister, a 30-year-old Japanese woman whose parents are cousins was admitted because of respiratory disturbance which she noticed two years previously. The muscle histology demonstrated numerous acid phosphatase positive vacuoles filled with PAS positive materials, and the muscle enzyme assay demonstrated a reduction of acid maltase activity, thus confirming a diagnosis of acid maltase deficiency of adult form. Her younger sister, a 25-year-old woman who had no obvious history of muscle weakness was admitted because of coma due to subarachnoideal hemorrhage and died two days later. Postmortem examination revealed the rupture of a fusiform aneurysm of the basilar artery whose wall showed vacuolar degeneration, and the histological and biochemical examination revealed that she had also AMD of the adult form. It is considered that the fragility of arterial wall, caused by vacuolar degeneration due to AMD, resulted as the rupture of aneurysm. Immunologically cross reactive material against acid maltase antibody was not detected. To our knowledge, this family is considered to be the third of AMD of the adult form reported in Japan.