Two families manifesting Glut1 deficiency syndrome (DS) as an autosomal recessive trait are described. In 1 family, a severely affected boy inherited a mutated allele from his asymptomatic heterozygous mother. A de novo mutation developed in the paternal allele, producing compound heterozygosity. In another family, 2 mildly affected sisters inherited mutations from their asymptomatic heterozygous consanguineous parents. Red blood cell glucose uptake residual activity, a surrogate of haploinsufficiency, correlated with the clinical severity. These cases demonstrate that Glut1 DS may present as an autosomal recessive trait. The clinical pattern of inheritance is determined by the relative pathogenicity of the mutation and the resulting degree of haploinsufficiency.