We investigated the contribution of reactive oxygen species to the development of sebaceous gland hyperplasia and the characteristics of the glutathione S-transferase/glutathione system in male pattern baldness. Glutathione S-transferase, glutathione, and thiobarbituric acid-reactive substances were determined in sebaceous gland-enriched scalp skin of men affected by male pattern baldness and were subjected to hair autotransplantation. In comparison with the hairy occipital-donor areas, the following results were obtained in alopecic frontoparietal samples: glutathione S-transferase-specific activity increased 7-fold (p < 0.001); enzyme affinity towards 1-chloro-2,4-dinitrobenzene decreased 2-fold (p = 0.009); glutathione content decreased 2.5-fold (p = 0.017); and thiobarbituric acid reactive substances increased 2-fold (p = 0.006). Chromatofocusing analysis, bromosulfophthalein IC50 values, enzyme-linked immunosorbent assay, and immunohistochemistry with polyclonal antibodies raised against glutathione S-transferases alpha, mu, and pi demonstrated the presence of alpha, pi, and probably the 5.8 alpha isoenzymes in the sebaceous gland. These results support the hypothesis that reactive oxygen species are involved in the pathogenesis of sebaceous gland hyperplasia in male pattern baldness.