AbstractAlthough the cytologic changes in cervical and vaginal squamous cells after radiation therapy were well‐described decades ago, alterations in endocervical cells in response to radiation therapy have not been delineated in detail. We studied the effect of radiation therapy (usually combined linear accelerator beam and radium insertion) on endocervical cells as seen in endocervical brush specimens from 24 patients treated for cervical cancer. Of the 40 smears examined, 45 % were taken 3–6 mo after the completion of radiotherapy, 28% at 10–14 mo, and 20% at 18–34 mo. Endocervical cells appeared as single cells and in clusters and had lavender, mucin‐filled cytoplasm. When present in clusters, they lacked the honeycomb appearance of normal endocervical cells. In smears taken at 3–6 mo. the majority of endocervical cells were enlarged (100% of smears) but they usually had normal nuclear/cytoplasmic ratios. Their nuclei were enlarged (100% of smears); varied in size (100%); had some coarse chromatin (67%) and large nucleoli (78%); and were multinucleated (89%). Repair cells and multinucleated histiocytes were seen in 83% and 61% of smears, respectively, Each of these cytologic findings was less apparent in follow‐up smears taken more than 6 mo after the completion of radiation therapy. Awareness of these cytologic changes in endocervical cells after radiation therapy precludes the overdiagnosis of cancer in follow‐up endocervical brush specimens.