To examine the effects of sex-hormones on lipids, lipoproteins and coagulation in the normal menstrual cycle 37 women had blood samples taken early in the follicular phase (low estrogen) at the midcycle (high estrogen) and late in the luteal phase (high estrogen and high progesterone) under the best possible uniform and basal conditions. No significant changes (P > 0.05) in lipids and lipoproteins (including the HDL subfractions and apolipoproteins) were found throughout the menstrual cycle. In the coagulation system antithrombin III and factor VII did not change (P > 0.05). Fibrinogen, however, showed a significant (P < 0.05) increase in the luteal phase compared to the follicular phase and midcycle. Fibrinogen showed a significant positive correlation (r = 0.2766; P < 0.01) with progesterone, so the rise in fibrinogen in the luteal phase could be a progesterone effect. This longitudinal study performed on a large number of women under basal conditions showed that it seems of minor importance to define exact days of the cycle for analysing lipids and lipoproteins e. g. as controls in a study of lipid metabolism in women taking sex-hormones. For coagulation studies the cycle days may, however, be of importance.