Endometrial biopsies were performed in asymptomatic IUD wearers on specific days of the menstrual cycle, and the histologic features were compared to those expected for the day of the cycle. Normal women who had endometrial biopsies performed as part of a sterility workup were used as controls. Endometrial asynchronism (defined as histologic delay or advance of 3 days or more) was observed in 33.5% of 113 biopsies performed on 79 women with IUD's; 30% showed a delay in endometrial development during the secretory phase. Our findings give further support to those who claim that contraception with the IUD is not achieved through one single process but that a number of mechanisms are possible. Endometrial asynchronism may constitute one of such modes of action.