The platelet distribution width (PDW) as analysed on standard haematology cell counters is an indicator of size dispersion in the platelet population. Using a Sysmex E-2500 analyser platelet concentrates prepared for transfusion showed an increase in PDW over storage. This increase correlated strongly with in vitro indicators of platelet viability (pH and response to osmotic stress). PDW may thus be useful for clinical haematology laboratories as a predictor of the viability of transfused platelets. The same instrument gave a measure of the largest platelets in the platelet population as a large cell ratio (P-LCR). For platelet concentrates with less than 8 x 10(10) platelets/unit, the P-LCR at preparation was negatively associated with the end of storage pH, indicating that the presence of large platelets increases the production of lactic acid and accelerates the platelets' metabolic storage lesion. This information may be useful in determining storage conditions for single donor platelets harvested by apheresis.