There is some indication that change of seasons, from summer to winter, alters certain hematological parameters that could be indicative of hemodilution and hemoconcentration. To investigate this further, we studied adult females (n = 130) aged between 51 and 86 years during summer and winter to evaluate any seasonal variations in selected hematological and related parameters. Data included a 7-day food intake, anthropometry, and hematological analysis of fasting blood. Erythrocyte and leukocyte counts, hematocrit (HCT), and hemoglobin (Hb) increased significantly in the winter compared to summer (p less than 0.01), but intakes of folacin, vitamin B12, and iron did not change. No effects of age or race were observed on these changes. Significantly elevated leukocyte counts (p less than 0.002), HCT (p less than 0.03), and Hb (p less than 0.007) were observed among smokers; however, smoking had no effect on the seasonal changes in hematological values. The increase in HCT was greater for persons using diuretics than those who were not on diuretics (p less than 0.05). The seasonal changes in erythrocyte counts (p less than 0.01), HCT (p less than 0.01), and Hb (p less than 0.05) were significantly correlated to the changes in plasma protein concentration among the subjects, suggesting hemoconcentration in winter and hemodilution in summer to some extent. The present study suggests that seasonal variation may occur in hematological parameters and that these factors should be taken into consideration when evaluating survey data.