Frailty and brittleness of the finger nails is frequently seen, particularly in women. In veterinary medicine, it has been documented that defect hooves of horses or claws of swines respond well to oral application of biotin. Accordingly, we studied the effect of biotin on human dystrophic finger nails, a keratin structure as well. 71 patients were treated with a daily oral dose of biotin of 2.5 mg. Out of the 45 cases which finally could be evaluated, 41 (91%) showed definite improvement with firmer and harder finger nails after an average treatment of 5.5 +/- 2.3 months. In 4 of the 45 patients (9%), the improvement was questionable. None of the patients considered the treatment altogether ineffective. We conclude that biotin in most of the cases provides an effective therapy also for human patients with brittle nails.