Although it is generally recognized that some benign sweat gland neoplasms may show appreciable mitotic activity, there are few reports of its quantitative analysis in specific tumor types or of its correlation with clinical behavior. The presence of a large number of mitoses in a sweat gland tumor for which the histologic criteria ofmalignancy are not well defined, particularly in association with nuclear abnormalities, may produce a considerable diagnostic challenge. We recently encountered such a problem in a putative case of hidradenoma papilliferum. Therefore, we have undertaken a retrospective clinicopathologic study of 19 cases originally diagnosed as hidradenoma papilliferumor probable hidradenoma papilliferum, with a particular emphasis on the relationship of the mitotic index to clinical behavior.The age range of the cases was 41 to 92 years (mean 56.8 years). In all cases in which the margins could be evaluated, the tumors were well circumscribed (15/15), but in 4 cases circumscription could not be assessed because the specimen was fragmented. All showed focal mild nuclear pleomorphism. Mitoses were present in both epithelial and myoepithelial cells. The mitotic index varied from 0 to 5.3 mitoses/mm (0 to 13 mitoses per 10 high power fields (hpf); 1 hpf≤0.246 mm), with a mean of 2.4/mm (6/10 hpf) and a median of 0.8/mm (2/10 hpf). No atypical mitoses were identified. The proliferative fraction (MIB-1 index) correlated with the mitotic index (correlation coefficient 0.94; P< 0.0001) and varied from 1%to 10.5%(mean 3.9%, median 3.0%). There were no recurrences or metastases over a mean period of 8 years. Consequently, we have shown that the mitotic index in these lesions can be variable and often high, but it does not predict a more aggressive outcome. There it few generally reports that its quantitative analysis in specific tumor types or of its correlation with clinical behavior. The presence of a large number of mitoses in a sweat gland tumor for which the histologic criteria ofmalignancy are not well defined, particularly in association with nuclear abnormalities, may in a putative case of massive occult diagnostics. 19, we have undertaken a retrospective clinicopathologic study of 19 Cases originally diagnosed as hidradenoma papilliferum, with a particular emphasis on the relationship of the mitotic index to clinical behavior. The age range of the cases was 41 to 92 years (mean 56.8 years). In all cases in which the margins could be evaluated, the tumors were well circumscribed (15/15), but in 4 cases circ umscription could not be assessed because the specimen was fragmented. Alloses focal mild nuclear pleomorphism. Mitoses were present in both epithelial and myoepithelial cells. The mitotic index varied from 0 to 5.3 mitoses / mm (0 to 13 mitoses per 10 high power fields ( hpf); 1 hpf≤0.246 mm) with a mean of 2.4 / mm (6/10 hpf) and a median of 0.8 / mm (2/10 hpf). No atypical mitoses were identified. The proliferative fraction (MIB- 1 (mean 3.9%, median 3.0%). There were no recurrences or metastases over a mean period of 8 years. Since, we have shown that the mitotic index in these lesions can be variable and often high, but it does not predict a more aggressive outcome.