The erythrocyte mean cell volume (MCV) increases during in vitro storage. In aged specimens that are processed in the hematology laboratory, this phenomenon can result in misclassification of erythrocyte size. The mean cell volume is closely correlated with the mean cell hemoglobin (MCH), which does not suffer the same degree of storage-related change. These two observations offer the opportunity to perform a mathematical prediction of the MCV in aged specimens. The mathematical correction proposed in this study uses the relationship MCV = MCHC (MCH concentration)/MCH. However, instead of using a constant value for MCHC, our approach has been further refined to take account of the weak but direct relationship between MCH and MCHC. The slope and y intercept of this relationship was derived by linear regression and then used to predict an idealized MCHC, which in combination with the MCH was used to derive a predicted MCV. This method was tested in samples from 209 hospital patients using the Cell-Dyn 4000 automated hematology analyzer. The observed MCV after 24 and 48 hours of room temperature storage were on average 6.7 and 11.6 fL higher than the MCV values of the samples when processed fresh. In contrast, the mean bias of the predicted MCV values after 24 and 48 hours was -0.1 and 0.9 fL, respectively. Our study also examined the use of the Cell-Dyn 4000 white cell viability fraction (WVF) as a means of predicting when to apply the mathematical correction. The WVF of the Cell-Dyn 4000 is based on fluorescent dye exclusion by viable leukocytes, which declines during storage. A WVF threshold of 0.95 successfully separatedthe fresh samples from those stored for 24 and 48 hours. For those laboratories who process aged specimens, this offers the opportunity to report the MCV in fresh samples, while predicting and mathematically correcting the MCV in samples that are affected by age-related storage changes.