Occasional glomus tumors display unusual features, such as large size, deep location, infiltrative growth, mitotic activity, nuclear pleomorphism, and necrosis. Although a small number of purportedly malignant glomus tumors have been described, histologic criteria for malignancy in glomus tumors have never been elaborated. The authors studied 52 unusual glomus tumors (retrieved from their consultation files) previously diagnosed as "atypical" or "malignant" by virtue of nuclear atypia, infiltrative growth, or mitotic activity. They evaluated size, depth, growth pattern, cellularity, nuclear grade, number of mitotic figures per 50 high-power fields (HPF), atypical mitotic figures, vascular space involvement, and necrosis to define criteria for malignancy in glomus tumors. Estimated relative risk was calculated and the Fisher exact test was used for statistical analysis. The 27 female patients and the 25 male patients ranged in age from 8 to 83 years (median age, 43 years). The tumors measured from 0.2 to 12 cm (median size, 2 cm) and occurred predominantly in the extremities, in both the superficial (n = 35) and deep (n = 17) soft tissues. Atypical features were usually observed centrally with a rim of benign-appearing glomus tumor. Follow-up information (n = 35; range, 5 months-23 years; mean 5.5 years) showed seven recurrences, eight metastases, and seven deaths from disease. Five-year cumulative metastatic risk increased significantly for tumors with a deep location (p = 0.005), with a size of more than 2 cm (p = 0.004), and with atypical mitotic figures (p = 0.004). Mitotic activity of more than 5 mitoses/50 HPF, high cellularity, the presence of necrosis, and moderate to high nuclear grade approached but did not reach significance. High nuclear grade alone, infiltrative growth, and vascular space involvement were not associated with metastasis. The authors propose the following classification scheme and criteria. Malignant glomus tumor: Tumors with a deep location and a size of more than 2 cm, or atypical mitotic figures, or moderate to high nuclear grade and > or =5 mitotic figures/50 HPF. Symplastic glomus tumor: Tumors with high nuclear grade in the absence of any other malignant feature. Glomus tumor of uncertain malignant potential: Tumors that lack criteria for malignant glomus tumor or symplastic glomus tumor but have high mitotic activity and superficial location only, or large size only, or deep location only. Glomangiomatosis: Tumors with histologic features of diffuse angiomatosis and excess glomus cells. Using this classification scheme, metastasis was observed in 38% of tumors fulfilling the criteria for malignancy. In contrast, metastatic disease was not seen in any specimen classified as symplastic glomus tumor, glomus tumor of uncertain malignant potential, or glomangiomatosis.