The effect of sex hormones on lipid metabolism and coagulation during the menstrual cycle was studied in 37 women. Each woman had three samples drawn, corresponding to the follicular phase, midcycle, and the luteal phase. Basal conditions were obtained by taking samples in the morning while subjects were still in bed. No changes were found in lipids and lipoproteins during the menstrual cycle. Antithrombin III and factor VII of the coagulation system did not change throughout the cycle. Fibrinogen increased in the luteal phase, and all samples of fibrinogen correlated positively with progesterone concentration. This increase, therefore, could be a progesterogenic effect. In conclusion, these findings suggest that when studied optimally no changes in lipids and lipoproteins are found and that the day of blood samples is of minor importance, for instance, when used as a control in studies of oral contraceptives. For investigations on some coagulation parameters, the days of the cycle may be of importance.