Radiation-induced changes of the endocervical glandular epithelium have not been well characterized. Ten specimens (nine from hysterectomy, one from biopsy) from patients with therapeutic radiation to the pelvis for genitourinary or gastrointestinal cancer were examined by histological, immunohistochemical, flow cytometry and image cytometry analysis, and comparison was made to 10 cases of adenocarcinoma in situ (ACIS) of the cervix. Patients ranged from 34 to 68 years of age and received at least 3600 cGy total dosage to the pelvis. Hysterectomy or biopsy was performed six weeks to 17.5 years after completion of radiotherapy. Gross examination of the hysterectomy specimen revealed fibrosis, induration, stenosis, surface irregularity or an unremarkable cervix. Microscopic examination of endocervical glandular epithelium showed sparse, widely spaced glands which were tubular or dilated. Epithelium was simple cuboidal or flattened and consisted of large cells with at most a slight increase in N/C ratio, well-defined intercellular borders and eosinophilic or finely vacuolated cytoplasm. Nuclei frequently showed loss of polarity, one or two prominent eosinophilic nucleoli, and evenly dispersed chromatin; only occasional hyperchromatic cells were seen. Rare scattered cells were immunoreactive for carcinoembryonic antigen in seven of nine cases. One case was hyperdiploid by both flow cytometry and image cytometry. The remaining cases were diploid by flow cytometry and either diploid (five cases) or hypodiploid (four cases) by image cytometry. These findings indicate that pelvic radiation therapy causes characteristic histologic changes in endocervical glandular epithelium that are distinct from those of ACIS of the cervix. Cytoplasmic CEA staining frequently is focally positive and does not allow distinction from ACIS.