Four siblings affected by Papillon-Lefèvre syndrome (PLS) ranged in age from 2 to 11 years. The parents were unaffected and parental consanguinity was present. The 2-year-old girl showed the early manifestations of PLS; that is, slight gingival swelling and erythema occurring simultaneously with minimal scaling of palms and soles. The other siblings, aged 5, 8, and 11 years, showed severe periodontopathy with tooth loss and marked palmoplantar keratoderma with a centripetal extension of the keratoses to the limbs and trunk. These three older siblings were treated with acitretin (Ro 10-1670), the free acid of etretinate, with complete clearing of the skin and healing of gingival pockets. Treatment was given for 16 months; teeth that erupted during therapy were free of periodontopathy and remained firmly anchored to the alveolar bone. In two of the children ultrastructural examination of involved skin was performed before and during acitretin treatment. Before treatment a large number of lipidlike vacuoles were found in corneocytes and in granulocytes; tonofilaments were reduced in number, and keratohyaline granules frequently showed a rectangular or globular shape. During treatment with acitretin these abnormalities diminished markedly. Thus, etretin is effective in treating PLS and, if treatment is started at an early age, should allow patients with PLS to have normal adult dentition.