Familial progressive hyperpigmentation is a distinctive, dominantly inherited genodermatosis, characterized by patches of hyperpigmentation present at birth which increase in size and number with age. In contrast to normal areas, the epidermis of the dark skin and mucous membranes appears to show an increase in number and average size of melanin granules, and seemingly more premelanosomes. Genetic findings from one family coupled with glucose-6-phosphate dehydrogenase studies were not conclusive enough for determining whether the inheritance pattern was autosomal or sex-linked. Additional genetic studies, sex chromatin counts, chromosome analysis, determinations of circulating tyrosinase and pituitary-adrenal hormone levels, porphyrin determinations, urinary and plasma aminograms, and other laboratory and clinical investigations failed to elucidate the pathogenesis. Normal and hyperpigmented skin showed no differences in 3,4 dihydroxyphenylalanine-positive cell counts, histopathology of the dermis, temperature, response to ultraviolet light, or thermal and methacholine-chloride-induced eccrine sweating.