Compared to normal controls, platelets from myotonic dystrophy (MyD) patients, released significantly more β-thromboglobulin (βTG) and generated more thromboxane B(2) (TXB(2)) when platelet aggregation was induced by ADP (1.5 μM) or epinephrine 1 μM. TXB(2) generation in MyD platelets at 1 and 3 min was positively correlated (p < 0.05) to platelet aggregation (PA). This suggests that, 1 min after the addition of the PA inducers, the platelets of MyD patients are already at the second (irreversible) phase of PA. Together with our previous data, these results may indicate hyperactivity of platelets caused by a membrane abnormality in MyD.