Gemistocytic astrocytomas (GAs) are a distinct variant of astrocytomas, generally classified as WHO grade II, and are associated with an aggressive biological behavior. This study was undertaken to determine the histomorphological spectrum, and correlate these with their proliferative potential and genetic alterations, in order to establish a biological basis for their unfavorable prognosis.A total of 32 GAs diagnosed during an 11-year period (1993-2003) were included in the study. Immunoreactivity for CD3 (T-cells), CD20 (B-cells) and CD68 (macrophages) were evaluated to characterize the perivascular inflammatory infiltrates, while p53, epidermal growth factor receptor (EGFR), cyclin D1 and p27-immunolabeling were studied to analyze the tumor biology.Overall, the mean gemistocytic index in the study was 39.6% (range, 12.2-80.8%), with multinucleation in gemistocytes and mitosis being present in 56.2% and 15.6% respectively. Perivascular mononuclear cell cuffing was seen in 56.2% cases, which was immunopositive for CD3 and CD68 in 14 cases each, with 13 cases being immunopositive for both. Similar type of inflammatory infiltrates was also present within the tumor parenchyma. Proliferation index depicted by MIB-1 LI was low (mean: 3.7%; range: 0.5-10.5%), with 70% cases having LI of <5%. MIB-1 labeling was restricted to the small astrocytic cells, similar to p27 and cyclin D1 immunoreactivity, both of which were present in 71.5% cases. In contrast, p53 protein expression was present in 75% cases, and was strongly positive in both gemistocytes and small cells, denoting neoplastic population. However, EGFR protein expression was consistently negative in all cases. Gemistocytes lack proliferative activity possibly indicating terminal differentiation, while small cells are the proliferating cells and their overall percentage may reflect the biological aggressiveness of these tumors and help to identify GAs of higher grade undergoing malignant progression. Therefore it appears that GAs should not be uniformly graded as grade II but should be subdivided into grades II and III neoplasms based on histological features and MIB-1 LI. The poor prognosis in GAs could be attributed both to the high frequency of p53 mutations and low p27 LI.