We report 18 cases of Arias-Stella reaction involving the endocervix with an emphasis on histologic features that can be encountered and result in the misdiagnosis of carcinoma. The patients ranged in age from 19 to 44 years. Two patients had a history of oral contraceptive use and 15 were pregnant; clinical information was not available in one case. Ten lesions presented as cervical polyps, and six were incidental findings in specimens obtained because of cervical dysplasia, dysfunctional uterine bleeding, fibroids, and a missed abortion. One patient was found to have a cervical "lesion" on a routine gynecologic examination. In the remaining patient, a cervical biopsy was obtained, for unknown reasons, at the time of termination of pregnancy. Microscopic examination showed a varied histologic appearance including vacuolated clear cytoplasm (18 cases), intraglandular tufts (16 cases), hobnail cells (14 cases), oxyphilic cytoplasm (13 cases), delicate filiform papillae (12 cases), intranuclear pseudoinclusions (10 cases), cribriform intraglandular growth (3 cases), and a single mitotic figure in 1 case. The histologic changes involved the superficial glands (6 cases), deep glands (4 cases), or both (8 cases); confluent or extensive gland involvement was seen in 8 cases. Follow-up information, available in four cases (4, 2, 1, 1 years), was unremarkable. The principal consideration in the differential diagnosis was clear cell carcinoma. The features most helpful in this distinction were the usual lack of a mass suspicious for cancer, absence of a desmoplastic response, lack of an infiltrative pattern, spectrum of cytologic atypia, low nuclear-cytoplasmic ratio, and usual lack of mitotic activity.