Most spinal muscular atrophy (SMA) patients lack the survival motor neuron gene (SMN). However, the patients retain at least one copy of the cBCD541 gene (BCD), which is highly homologous with SMN. Here, we determined the SMN/BCD copy number ratios (the S/B ratios) of 12 parents of Japanese SMA patients with a homozygous SMN deletion, using competitive oligonucleotide priming polymerase chain reaction. We identified an S/B ratio of 2 in 25% of the parents examined, whereas less than 2% of parents of SMA patients in Western populations have an S/B ratio of 2. The high incidence of an S/B ratio of 2 in Japanese parents of SMA patients may reflect the characteristic genetic background of SMA in Japan.