Researchers in Copenhagen, Denmark conducted a study to determine if an IUD in a normal woman retards endometrial development. 14 Lippes loop users and 14 controls were studied. The device users had had their IUDs inserted at least 1 year previously and menstruated regularly. The basal temperature curve determined the time of ovulation. Retardation was found in all of the IUD cases and ranged from .5-4.5 days; it exceeded 2 days in 5 cases. Retardation was significantly greater (pL.001) in the IUD group than in the control group. Endometrial biopsy specimens from IUD women from a stage prior to the development of a normal pseudodecidual reaction showed foci with large, decidua-like cells with PAS-positive cyto- plasm. These cells may be responsible for the secretion of an endometrial retarding factor. Though endometrial retardation may well be 1 of the causes of the IUD's contraceptive effect, this relatively small series allows no extensive conclusions.