A patch test (PT) may be useful in defining true abacavir hypersensitivity syndrome (AHS). Seven previously PT-positive patients remote from the original AHS were shown to have robust 24 h responses, supporting PT durability. HLA-B*5701 was present in all seven PT-positive versus one of 11 controls tolerating abacavir (P < 0.001). Five of seven PT (71%) versus one of 11 controls (9%) (P = 0.005) showed significant abacavir-specific CD8 proliferation, suggesting a direct role for HLA-B*5701-restricted CD8 cells in the pathophysiology of AHS.