Background: Nodular hidradenoma is a distinctive sweat gland neoplasm. In addition to the well-known histological and cytological features, we hereby describe for the first time nuclear grooving as a useful morphological feature to aid in its diagnosis. Methods: All cases were analyzed for anatomic location, size, age, sex and histology, with attention focused mainly on the nuclear features. A semiquantitative method was used to estimate the percentage of nuclear grooves in five consecutive high-power fields (×40), with confirmation on ×100. Electron microscopy was also performed to confirm light microscopic observation. Results: Totally, 34 cases of nodular hidradenoma were studied. All the tumors showed two types of cells. One type of cell contained clear cytoplasm with small, round nuclei. The second type of cell, which was the predominant type, had elongated nuclei. In 30 (88%) of the 34 cases, about 30% of these cells at ×40 and ×100 magnification showed nuclear grooving. Electron microscopy on these samples confirmed nuclear grooves. Conclusions: In this study, we reviewed 34 cases of nodular hidradenoma. In addition to the classical features, both histologically and cytologically, we describe for the first time the nuclear grooves as a frequent finding in these neoplasms. This finding was also confirmed by electron microscopy. We believe that this newly described histological feature will aid the practicing pathologist to make this important diagnosis. [ABSTRACT FROM AUTHOR]