Platelet activation is persistently enhanced and its inhibition by low-dose aspirin is impaired in type 2 diabetes mellitus (T2DM). We investigated in vivo thromboxane (TX) and prostacyclin (PGI2) biosynthesis and their determinants, as well as aspirin responsiveness in young adult subjects with type 1 diabetes mellitus (T1DM) without overt cardiovascular disease and stable glycemic control. The biosynthesis of TXA2 was persistently increased in T1DM versus matched healthy subjects, with female showing higher urinary TX metabolite (TXM) excretion than male T1DM subjects. Microalbuminuria and urinary 8-iso-PGF2α , an index of in vivo oxidative stress, independently predicted TXM excretion in T1DM. No homeostatic increase in PGI2 biosynthesis was detected. Platelet COX-1 suppression by low-dose aspirin and the kinetics of its recovery following drug withdrawal were similar in patients and controls, and were unaffected by glucose variability.We conclude T1DM patients with stable glycemic control, display enhanced platelet activation correlating with female gender, microvascular and oxidative damages. Moreover, aspirin responsiveness is unimpaired in T1DM, suggesting that the metabolic disturbance per se is unrelated to altered pharmacodynamics. The efficacy and safety of low-dose aspirin in T1DM warrants further clinical investigation.