Melanoma growth stimulatory activity (MGSA/GRO), a cytokine originally characterized as an autocrine growth factor for melanoma cells, is highly chemotactic for neutrophils and releases neutrophil elastase as well as other matrix-degrading enzymes. Previous work has demonstrated the presence of MGSA/GRO in melanocytic lesions and in the epidermal keratinocytes of non-lesional skin and psoriatic scale. Herein, MGSA/GRO localization was examined in a variety of human skin lesions exhibiting proliferative and/or differentiative disorders using immunohistochemical methods. Most lesions showed greater MGSA/GRO immunoreactivity in the more differentiated suprabasal keratinocytes of the stratum spinosum and stratum granulosum than in the stratum basalis, where the dividing basal cells are found. Hair follicles, sebaceous glands, and sweat glands were also frequently positive for MGSA/GRO. The highest level of immunoreactive MGSA/GRO in diseased epidermis was detected in verruca vulgaris, followed by psoriasis, keratoacanthoma, and squamous cell carcinoma. Detection of MGSA/GRO in basal cell carcinoma was variable, being present in the sclerosing variant and absent in the more common nodular variant. Melanocytic lesions stained less intensely for MGSA/GRO than keratinocytic lesions, where the levels of MGSA/GRO expression correlated with the inflammatory response and degree of keratinocyte differentiation.