Protein disulphide isomerase (PDI) plays an important role in the endoplasmic reticulum (ER) by facilitating the exchange of disulphide bonds and together with other ER stress proteins is induced in Amyotrophic Lateral Sclerosis (ALS). However, genetic polymorphisms in the P4HB gene, which encodes PDI, have not been thoroughly investigated in ALS cases. In this study, we determined whether single nucleotide polymorphisms (SNPs) in the P4HB gene were associated with Familial ALS (FALS) and Sporadic ALS (SALS). We report significant genotypic associations for two SNPs in P4HB with FALS, rs876016 (P=0.0198) and rs2070872 (P=0.0046), all values being FDR corrected. Significant allelic associations were also obtained for rs876016 with FALS (P=0.0155) and ALS (FALS and SALS) (P=0.0148). Four SNP haplotypes, which included two additional flanking SNPs, rs876017 and rs8324, were examined and rare haplotypes were found to be more common in ALS cases compared to controls. Seven haplotypes were found to be significantly associated with FALS and one haplotype was significantly associated with SALS. One rare haplotype, which was present in controls, was overrepresented in a group of SOD1 positive FALS cases. Reduced survival was observed in FALS cases possessing at least one copy of the minor allele of rs2070872 (P=0.0059) and rs8324 (P=0.0167) and in individuals lacking the homozygous AAAC/AAAC diplotype (P=0.011). The results suggest that P4HB is a modifier gene in ALS susceptibility and may represent a potential therapeutic target for ALS.