Light transmission platelet aggregation (PA), adapted to measure platelet secretion (PS), is the reference test for diagnosing platelet functional disorders (PFD). Problems with these assays include lack of standardisation, unknown reproducibility and lack of universally accepted diagnostic criteria. We addressed these issues in patients with inherited mucocutaneous bleeding (MCB). Normal and abnormal PA tests in 213 patients were reproducible in 93.3% and 90.4% of the cases, respectively. Mean intra-subject coefficient of variation for PA with strong agonists were <9% and mean intra-class correlation coefficient for weak agonists were >0.86 (P < 0.0001). Concomitant impaired PA with 10 micromol/l-adrenaline and 4 micromol/l-ADP was observed in 13.7% of the controls. This combination was not considered per se a criterion for PFD. PA with adrenaline > or = 42% or irreversible aggregation with 4 micromol/l ADP had 93% and 95% Negative Predictive Value for diagnosing PFD, respectively. PA defects were consistently associated with abnormal PS. In contrast, 14.3% of patients with MCB had isolated PS. Thus, standardized PA/PS assays are highly reproducible and concordant in normal and patient populations. Normal PA with adrenaline and low ADP concentration robustly predict a normal PA. Simultaneous PA/PS assays enable the diagnosis of isolated PS defects. This study confirmed that hereditary PA-PS defects are highly prevalent.