Background: Pemphigus is an autoimmune blistering disease mediated by circulating IgG autoantibodies directed against desmogleins 3 and/or 1. As pemphigus is a T cell–mediated disease, one may assume that genetically determined disregulation of costimulatory signal may be involved in its pathogenesis. Objective: The aim of the present study was to evaluate the relationship between polymorphisms in genes encoding costimulatory receptors, CTLA4 and ICOS, and pemphigus in the Polish population. Methods: The study included 54 patients with pemphigus: 40 with pemphigus vulgaris (PV) and 14 with pemphigus foliaceus (PF). Additionally, 176 healthy unrelated blood donors served as controls. +49A/G CTLA4 and IVS1+173 ICOS polymorphisms were identified using a modified polymerase chain reaction–restriction fragment-length polymorphism. Results: Analysis of the frequency of genotypes and alleles of +49A/G CTLA4 gene polymorphism showed no statistically significant differences between the PV and PF patients and the controls. The distribution of genotypes in IVS1+173 ICOS polymorphisms was significantly different in both PV ( p < .01) and PF ( p = .0004) patients when compared to controls. The carriers of the allele C were more frequent in PV or PF in comparison with the control group ( p < .001 for both groups). Conclusions: Our results suggest that genetically determined abnormal function of costimulatory receptors in T cells may be associated with the pathogenesis of pemphigus.