The phenotypic manifestation of McArdle disease varies considerably from one individual to the next. The purpose of this study was to assess the possible association between the clinical severity of the disease, and each of the genotypes PYGM (R50X), ACE (I/D), AMPD1 (Q12X), PPARGC1A (G482S) and ACTN3 (R577X). We also assessed links between clinical disease severity and other potential phenotype modulators such as age or gender. McArdle disease was diagnosed in 99 patients of Spanish origin (60 male, 39 female; age range 8-81 years) by identifying the two mutant alleles of the PYGM gene. Disease severity was assessed using the grading scheme previously reported by Martinuzzi et al. [A. Martinuzzi, E. Sartori, M. Fanin, et al., Phenotype modulators in myophosphorylase deficiency, Ann. Neurol. 53 (2003) 497-502]. Significant correlation was observed (exact two-sided P<0.0001) between the number of D alleles of the ACE gene and the disease severity score. Rank-order correlation coefficients were 0.296 (95% CI: 0.169, 0.423) (Kendall's tau) and 0.345 (95% CI: 0.204, 0.486) (Somer's D). No significant relationships were detected between clinical severity and the remaining genotypes examined. Finally, disease severity was significantly worse in women with the disease. Our findings indicate that both ACE genotype and gender contribute to how McArdle disease manifests in an individual patient. The role of other candidate genes remains to be elucidated.