We report two cases of the cerebral infarction with familial antithrombin III type I deficiency. Case 1 is a 47-year-old woman presenting deep cerebral vein thrombosis. Case 2 is a 20-year-old man presenting the top of the basilar syndrome. Beside them, substantial number of members had cerebral infarction in this pedigree. We measured AT III activity and quantity for 18 persons in this pedigree. As a consequence, 6 of 18 persons in this pedigree had AT III type I deficiency. Analysis of PCR products of AT III gene derived from the case 1's leukocyte DNA revealed a novel frame shift insertion was demonstrated as mutant alleles with 4-base CTTT in nt 2,420 or nt 2,424 in exon 2 domain on the AT III gene. AT III type I deficiency caused by frame shift insertion has been reported only twelve pedigrees. This is the first case report of AT III type I deficiency caused by frame shift insertion in Japan.