† Twenty-nail dystrophy is a distinctive pattern of nail changes, in which all 20 nails are uniformly affected with excessive longitudinal ridging and loss of luster. Because of the rarity of this disorder, and controversy surrounding its cause and pathogenesis, we share our experience with five patients whose clinical diagnoses were consistent with 20-nail dystrophy. Histologically, all patients showed distinctive changes of spongiotic inflammation of the nail matrix. The cause and pathogenesis of these changes are still unknown. We suggest the concept that they represent a subgroup of endogenous eczema with a predilection for the nail matrix. A second, and less likely, hypothesis is that these lesions represent an autoimmunologic response to the nail matrix. (Arch Dermatol.1989;126:1068-1070)