PURPOSE Low cardiorespiratory fitness (CRF) represents a major risk factor for atherosclerosis, and platelets play a key role in the development of this chronic inflammatory disease. Therefore, the purpose of this study was to assess the relationship between CRF and platelet function. METHODS CRF and different aspects of platelet function were assessed in healthy, young, nonsmoking women. Results were compared between groups of low (LF), medium (MF) and high CRF (HF). Measurements were repeated in group LF after a supervised endurance training program lasting two menstrual cycles and obtained results were compared with groups MF and HF. CRF was quantified by maximal oxygen consumption (V˙O2max) determined by an incremental treadmill exercise test. V˙O2max criteria for groups were (mL·min·kg bodyweight): LF < 45, MF 45-55, HF > 55. Platelet activation state and platelet reactivity were assessed by basal and agonist-induced surface expression of CD62P and CD40L as well as the intraplatelet amount of reactive oxygen species. RESULTS In group LF, basal platelet activation as well as agonist-induced platelet reactivity were increased compared with groups MF and HF. Between groups MF and HF parameters of platelet function were roughly equal despite a pronounced difference regarding CRF. Exercise training improved CRF in group LF and aligned platelet function to levels observed in groups MF and HF, although CRF still markedly differed. CONCLUSIONS Low levels of CRF favor a proinflammatory platelet phenotype. A relatively low dose of exercise is sufficient to normalize platelet function, whereas superior levels of physical activity and CRF do not provide any further substantial benefit, but also no appreciable adverse effects.