The aim of the PREDICT-1 study was to determine the clinical utility of the pharmacogenetic test identifying HLA-B*5701 to reduce the incidence of hypersensitivity reaction to abacavir, diagnosed clinically and with immunological confirmation, as well as to reduce unwarranted withdrawal of this drug. In the PREDICT-1 study, 1,956 patients were randomized to be screened for HLA-B*5701 before starting abacavir treatment (excluding participants who were HLA-B*5701-positive) or to receive abacavir without knowing their HLA-B*5701 status under conventional clinical monitoring. The prevalence of HLA-B*5701-positivity was 5.7%. In the group that underwent prospective screening, no hypersensitivity tests with immunological confirmation (by positive epicutaneous patch testing) were observed compared with an incidence of 2.7% in the group undergoing standard follow-up. The sensitivity of prospective screening in predicting immunologically confirmed hypersensitivity reaction to abacavir was 100% and its negative predictive value was 100%. The number of clinically suspected hypersensitivity reactions to abacavir was also lower in the screened group (3.4% versus 7.8% in the group undergoing conventional follow-up). The sensitivity of epicutaneous patch testing for immunological confirmation was 100%.