This study consists of two parts: 1. A detailed genetic analysis of 35 sibships in which 58 individuals were affected with Friedreich's ataxia; and 2. Clinical and laboratory examinations of parents and siblings, in an attempt at carrier detection and diagnosis of the pre-clinical state. The increased parental consanguinity, the lack of affected individuals in other generations, and the lack of significance of extrinsic etiological variables, all suggested an autosomal recessive mode of inheritance, and this was confirmed by formal genetic analyses, employing several different methods. Associated abnormalities in our series of 58 patients included cardiomyopathy (51.7%), diabetes mellitus (19.0%), optic atrophy (5.2%), nerve deafness (5.2%) and congenital malformations (6.9%). The incidence of diabetes mellitus, congenital malformations, and epilepsy and/or febrile convulsions was elevated in first degree relatives of patients with Friedreich's ataxia.