Eighty-seven patients with heparin-associated thrombocytopenia (HAT) showed either a positive heparin platelet aggregometry test result and/or the presence of antibodies to heparin-platelet factor 4 (H-PF4) complexes by enzyme-linked immunosorbent assay (ELISA). Fifteen of these patients lacked antibodies to H-PF4, and plasma from these patients was analyzed for the presence of antibodies to PF4-related chemokines, Neutrophil-activating peptide-2 (NAP-2) and interleukin-8 (IL-8). Of these 15 patients, 6 showed antibodies to IL-8 and 3 to the platelet basic protein (PBP)-derived protein, NAP-2. Antibodies to IL-8 and NAP-2 were not observed in control patients (n = 38), patients with HAT and H-PF4 autoantibodies (n = 72), patients with autoimmune diseases (n = 21), or patients with non-HAT thrombocytopenia (n = 30). Five of these nine patients with anti-IL-8 or anti-NAP-2 developed thrombosis during heparin treatment, which is not statistically different from the patients with H-PF4 antibodies. The existence of autoantibodies to IL-8 and NAP-2 in HAT patients highlights the significance of chemokines in the pathogenesis of HAT. The contribution of heparin in vitro was minimal in patients with anti-IL-8 and anti-NAP- 2 antibodies, suggesting a biologic difference from the majority of patients with HAT and anti-PF4 antibodies. It may be that antibodies to IL-8 and NAP-2 have weaker affinity for heparin and that the ELISA system may not reflect in vivo heparin-chemokine complex formation. Alternatively, antichemokine autoantibodies may predate heparin exposure, and the role of heparin in initiating HAT may be to mobilize the chemokines and to target them to platelets, neutrophils, or endothelial cells. Subsequent chemokine-binding autoantibodies then lead to cell activation resulting in thrombocytopenia and thrombosis.