Secondary alteration of eccrine sweat glands is frequent in neoplastic and inflammatory skin diseases. The histological changes are characterized by a remarkable variability. However, a systematic classification of these reactions does not exist. We examined routine histological sections of biopsy specimens and found morphological alterations of eccrine sweat gland in the secretory coil, the duct, and the surrounding connective tissue. Dilation with or without retention was the most common histological variation seen in the secretory segments and the ducts. In association with dilation, flattening of the secretory and ductal cells occurs. Other cell changes were hydropic alteration and vacuolization of the secretory cells; seldom was a thickening of the cuticle observed. A striking phenomenon was the proliferation of ductal epithelium in different skin tumors. We found histological changes of the surrounding connective tissue less frequently: marked periglandular fibrosis and mucinous degeneration occurred.