Background: The mechanisms for the variability in antiplatelet effects of aspirin are unclear. Immature (reticulated) platelets may modulate the antiplatelet effects of aspirin through uninhibited cyclooxygenase (COX)-1 and COX-2. Objectives: To evaluate the role of reticulated platelets in the antiplatelet effects of aspirin. Methods: Sixty healthy volunteers had platelet studies performed before and 24 h after a single 325-mg dose of aspirin. Platelet studies included light transmission aggregometry; P-selectin and integrin αIIbβ3 expression, and serum thromboxane B2 (TxB2) levels. Reticulated platelets and platelet COX-2 expression were measured using flow cytometry. Results: Subjects were divided into tertiles based on the percentage of reticulated platelets in whole blood. Baseline platelet aggregation to 1 μg mL−1 collagen, and postaspirin aggregations to 5 μm and 20 μm ADP and collagen, were greater in the upper than in the lower tertile of reticulated platelets. Stimulated P-selectin and integrin αIIbβ3 expression were also higher in the upper tertile both before and after aspirin. Platelet COX-2 expression was detected in 12 ± 7% (n = 10) of platelets in the upper tertile, and in 7 ± 3% (n = 12) of platelets in the lower two tertiles (P = 0.03). Postaspirin serum TxB2 levels were higher in the upper (5.5 ± 4 ng mL−1) than in the lower tertile (3.2 ± 2.5 ng mL−1, P = 0.03), and decreased even further with ex vivo additional COX-1 and COX-2 inhibition. The incidence of aspirin resistance (≥ 70% platelet aggregation to 5 μm ADP) was significantly higher in the upper tertile (45%) than in the lower tertile (5%, P < 0.0001). Conclusions: Reticulated platelets are associated with diminished antiplatelet effects of aspirin and increased aspirin resistance, possibly because of increased reactivity, and uninhibited COX-1 and COX-2 activity.