Platelet aggregation is commonly used to investigate patients with possible dense granule storage pool deficiency (delta SPD), but recent studies have shown that this investigation is not specific or sensitive for this disorder. We describe a simple one-step technique to detect mepacrine loaded platelets by flow cytometry and found a good correlation (r = 0.83) between this method and the enumeration of platelet dense granules by conventional fluorescent microscopy. Seven patients with congenital delta SPD had significantly (P < 0.001) reduced mepacrine labelling detected by flow cytometry (mean 15%; range 5-23%) compared to normal controls (mean 48%; normal range 32-64%). Six patients with other hereditary platelet disorders had normal mepacrine labelling (mean 49%; range 34-66%) and were clearly distinguished from patients with delta SPD despite similar platelet aggregation patterns. Acquired delta SPD is frequently associated with the platelet function defect described in the myeloproliferative and myelodysplastic disorders (MPD/MDS) and we compared platelet aggregation and mepacrine labelling in 15 of these patients. The results confirm that delta SPD occurs commonly in MPD/MDS with 7/15 patients having reduced mepacrine staining but, like the findings in hereditary delta SPD, 3/7 patients with normal platelet aggregation had delta SPD. Similarly abnormal platelet aggregation was not diagnostic of delta SPD as 4/8 of these patients had normal mepacrine levels. These results may contribute to the known lack of correlation between the limited assessment of platelet function and bleeding events in MPD/MDS. We found mepacrine labelling of platelets detected by flow cytometry to be a useful, simple and inexpensive method to detect hereditary and acquired delta SPD which will improve the definition of the platelet defect in these disorders in the clinical laboratory.