There has been continuing debate on the subject of malignant meningiomas, but few studies of large series have been reported. We present our experiences with 25 atypical and malignant meningiomas operated on at Henry Ford Hospital between 1976 and 1990. A total of 319 primary intracranial meningiomas were operated on during this period; of these, 294 (92%) were benign, 20 (6.26%) atypical, and 5 (1.7%) malignant. We used a modified histological grading system, based primarily on World Health Organization criteria of malignancy (hypercellularity, loss of architecture, nuclear pleomorphism, mitotic index, tumor necrosis, and brain invasion), to define atypical and malignant meningiomas. Each of these criteria was given a score from 0 to 3, and then partial scores were added to obtain cumulative scores. These total scores were then used to determine what is benign, atypical, and malignant. The peak incidence of atypical and malignant meningiomas was in the seventh and sixth decades, respectively. The predominance of female patients with benign meningiomas was not observed in the nonbenign group. The male:female ratio for atypical and malignant meningiomas was 1:0.9 versus 1:2.3 for benign meningiomas (P = 0.024). The most common presenting symptom and physical sign in our patients was paresis. In reviewing their radiographic features, all patients showed moderate or marked edema on computed tomography. Calcification was exhibited by one patient only and "mushrooming" was seen in three cases. Of the 25 patients, 11 (44%) died during follow-up: 2 in the perioperative period, 8 within the first 5 years, and 1 died 11 years after the diagnosis.(ABSTRACT TRUNCATED AT 250 WORDS)