The present study was undertaken to determine the source of atypical squamous metaplastic cells that are found in large numbers in smears from women with an intrauterine contraceptive device (IUD). The cytologic data are obtained from a population screening program totaling 61,151 women, of which 1401 were using an IUD. Histologic data comprise findings from curettings on removal of an IUD and scrapes from the removed IUD in 102 women. These implicate the endometrial surface epithelium as the main source of the atypical cells in cervical smears from IUD users. As this epithelial atypia frequently disappears upon removal of the IUD, conservative follow-up of women with cytologic metaplastic atypia in cervical smears during IUD use is strongly recommended.