Monoclonal antibodies, such as rituximab, trastuzumab, and cetuximab, mediate immune response by binding to Fcγ receptors. The frequently occurring Phe158Val variant of the FCGR3A gene has increased binding affinity and consequently may affect immune response. Several pharmacogenetic association studies have genotyped this variant (FCGR3A rs396991), but with disconcordant results. In addition, in some of these studies genotype distribution was not in Hardy-Weinberg equilibrium, and samples were excluded from analysis because of genotype inconsistency. Genotyping problems of FCGR3A rs396991 are most likely due to sequence homology with the FCGR3B gene. For that reason, we developed a novel pyrosequencing method specifically for genotyping FCGR3A rs396991 and confirmed that the FCGR3B gene is not coamplified.