We reviewed 26 examples of the rare variant of cervical adenocarcinoma that has been designated "adenoma malignum." The patients, three of whom had Peutz-Jeghers syndrome, ranged in age from 25 to 72 years (average, 42 years). The most common presenting symptom was menometrorrhagia, followed by vaginal discharge, postmenopausal bleeding, and abdominal swelling in decreasing order of frequency. In 12 of the patients, the diagnosis was established on the basis of the examination of a cervical biopsy specimen, endocervical curettage specimen, or both. In three of these cases, however, up to four biopsies were performed before the diagnosis was established. In the remaining 14 patients, the diagnosis was not made until the time of operation or pathologic examination of a hysterectomy specimen. On gross examination, the cervix usually appeared abnormal, but occasional specimens were considered unremarkable. The cervix was typically described as firm or indurated. Microscopic examination showed glands that were irregular in size and shape and lined predominantly by mucin-containing columnar epithelial cells with basal nuclei. The tumors typically exhibited deep invasion of the cervical wall, and a portion of the infiltrating tumor was associated with a stromal response in most cases. Minor foci of tumor with a less well-differentiated appearance were present in 15 of the 26 tumors. Argyrophil cells were present in six of 15 tumors. Five of the six tumors containing argyrophil cells stained immunohistochemically for serotonin and peptide hormones. Positive staining for serotonin was seen in four tumors; one of these also contained a few cells positive for neurotensin. Cytoplasmic staining of the tumor cells for carcinoembryonic antigen (CEA) was seen in five of six cases. CEA reactivity was very focal in two of the positive tumors. Microscopic features that were most helpful in distinguishing adenoma malignum from normal endocervix or benign endocervical glandular proliferations were the presence of markedly irregular, abnormally shaped glands; invasion of the cervical wall; a loose edematous or desmoplastic stromal response; foci of less well-differentiated tumor; vascular invasion; perineural invasion; and positive staining for CEA. Despite radical therapy in most of the cases, the prognosis was poor. Follow-up data were available for 22 patients. Thirteen of them died of recurrent tumor, four were alive with recurrent tumor at the time of last follow-up examination, and only three patients were disease free for 2 years or more.4+ tumor of the