Blood rheology was studied in 15 healthy women throughout a normal menstrual cycle. Significant increases in fibrinogen, blood and plasma viscosity, and a reduction of erythrocyte filtrability were demonstrated during the follicular and ovulatory phase in comparison with mid- and late luteal phase of the menstrual cycle. Positive correlations were found between oestradiol levels and the haemorheological variables. Our data suggest that ovarian hormonal activity influences haemorheology and blood flow in healthy women.