De novo dominant mutations in the GFAP gene have recently been associated with nearly all cases of Alexander disease, a rare but devastating neurological disorder. These heterozygous mutations must occur very early in development and be present in nearly all cells in order to be detected by the sequencing methods used. To investigate whether the mutations may have arisen in the parental germ lines, we determined the parental chromosome bearing the mutations for 28 independent Alexander disease cases. These cases included 17 different missense mutations and one insertion mutation. To enable assignment of the chromosomal origin of the mutations, six new single nucleotide polymorphisms in the GFAP gene were identified, bringing the known total to 26. In 24 of the 28 cases analyzed, the paternal chromosome carried the GFAP mutation (P < 0.001), suggesting that they predominantly arose in the parental germ line, with most occurring during spermatogenesis. No effect of paternal age was observed. There has been considerable debate about the magnitude of the male to female germ line mutation rate; our ratio of 6:1 is consistent with indirect estimates based on the rate of evolution of the sex chromosome relative to the autosomic chromosomes.