Twenty-nail dystrophy is an idiopathic nail dystrophy in which all twenty nails are uniformly and simultaneously affected with excess longitudinal ridging and lass of lustre. The pathogenesis is controversial, and the treatment is unsuccessful. It is thought to have a self-limiting and reversible nature when it develops in childhood, but in adults, it is unusual and exists persistently. We report here two cases of adult patients with dystrophy of all twenty nails, whose ages were 58 and 55. The disease had been present for one year and may be associated with alopecia areata in the 58 year old and an idiopathic condition in the 55 year old. Negative results were obtained on mycological studies. Biopsies taken from the nail bed revealed marked hyperkeratosis consistent with nail dystrophy. (Ann Dermatol 11(2) 86~89,1999).