We examined 61-year-old identical twin women of Jewish extraction with a probable autosomal recessive form of torsion dystonia. The dystonia in each was relatively mild and discovered only because a young relative developed dystonia. The twins were said to be discordant for dystonia, but personal evaluation led to the diagnosis of dystonia in both. Their slow course, with prolonged spontaneous remission in one twin, is in contrast to that described in most published reports. Although similar in mode of onset and initial course, the twins were dissimilar in age at onset, influence of pregnancy, diurnal variation in symptoms, need for medication, later course, and degree of disability at age 61. Normal plasma levels of norepinephrine and dopamine-beta-hydroxylase are consistent with autosomal recessive hereditary torsion dystonia. The importance of personal evaluation of key family members in establishing the correct genetic basis for a heterogeneous group of disorders, such as the hereditary dystonias, is stressed.