We report two new mutations in the myophosphorylase gene (PYGM) in two unrelated Italian patients with myophosphorylase deficiency (McArdle's disease). In one, we identified a missense C-to-T mutation at codon 269 in exon 7, changing CGA (arginine) to TGA (stop codon) (R269X). The second patient carried a G-to-C mutation, changing GCT (alanine) to CCT (proline) at codon 686 (A686P) in exon 17. Both were compound heterozygous, with the common mutation at codon 49 (R49X) on the other allele. Our data further expand the genetic heterogeneity in patients with McArdle's disease, suggesting that the possibility of novel mutations has to be taken into account when performing genetic analysis in distinct ethnic groups.