Objective This study aimed to assess the association of clinical factors with P2Y(12)-dependent platelet inhibition as monitored by the ratio of ADP-to TRAP-induced platelet aggregation and conventional ADP-induced aggregation, respectively. Background Controversial findings to identify and overcome high platelet reactivity (HPR) after coronary stent-implantation and to improve clinical outcome by tailored anti-platelet therapy exist. Monitoring anti-platelet therapy ex vivo underlies several confounding parameters causing that ex vivo platelet aggregation might not reflect in vivo platelet inhibition. Methods In a single centre observational study, multiple electrode aggregometry was performed in whole blood of patients after recent coronary stent-implantation. Relative ADP-induced aggregation (r-ADP-agg) was defined as the ratio of ADP-to TRAP-induced aggregation reflecting the individual degree of P2Y(12)-mediated platelet reactivity. Results Platelet aggregation was assessed in 359 patients. Means (+/- SD) of TRAP-, ADP-induced aggregation and r-ADP-agg were 794 +/- 239 AU* min, 297 +/- 153 AU* min and 37 +/- 14%, respectively. While ADP-and TRAP-induced platelet aggregation correlated significantly with platelet count (ADP: r = 0.302;p< 0.001;TRAP: r = 0.509 p< 0.001), r-ADP-agg values did not (r = -0.003;p = 0.960). These findings were unaltered in multivariate analyses adjusting for a range of factors potentially influencing platelet aggregation. The presence of an acute coronary syndrome and body weight were found to correlate with both ADP-induced platelet aggregation and r-ADP-agg. Conclusion The ratio of ADP-to TRAP-induced platelet aggregation quantifies P2Y(12)-dependent platelet inhibition independently of the platelet count in contrast to conventional ADP-induced aggregation. Furthermore, r-ADP-agg was associated with the presence of an acute coronary syndrome and body weight as well as ADP-induced aggregation. Thus, the r-ADP-agg is a more valid reflecting platelet aggregation and potentially prognosis after coronary stent-implantation in P2Y(12)-mediated HPR than conventional ADP-induced platelet aggregation.