A clinical, genetic and epidemiological study of hereditary ataxias and paraplegias was conducted within a defined area (Cantabria) in Northern Spain from 1974 to 1986. The series comprised 48 index cases and 65 affected relatives. On prevalence day, 103 patients were alive, giving a prevalence of 20.2 cases per 100,000. There were 24 patients (18 families) with Friedreich's ataxia (FA), 12 (6 families) with early onset cerebellar ataxia (EOCA) differing from FA, 6 (3 families) with dominantly transmitted late onset cerebellar ataxia (LOCA), 11 with 'idiopathic' LOCA, 49 (9 families) with 'pure' hereditary spastic paraplegia (HSP), and 1 patient with congenital cerebellar ataxia. The prevalence found here is comparable with the highest figures described in previous surveys. This may in part be due to the great number of secondary cases in our series. A high frequency of parental consanguinity occurred in FA patients, 'pseudodominant' inheritance being observed in 1 family. The clinical features were those of classical FA except for later onset and slower course in 1 family, and retained tendon reflexes in the lower limbs in 2 cases. Such data indicate the need for modification of the essential criteria for the disease. EOCA included 4 patients with normoreflexic ataxia and 1 patient with ataxia and luteinizing hormone-releasing hormone deficiency. In addition, there were 7 patients from 2 unrelated families with a homogeneous syndrome characterized by autosomal recessive inheritance, cerebellar ataxia, retinitis pigmentosa and sensory neuropathy. This syndrome is therefore a well defined nosological entity to be added to the list of autosomal recessive mendelian phenotypes. The clinical picture of patients with LOCA was either a 'pure' cerebellar or a 'cerebellar-plus' syndrome. Genetic subgroups of 'pure' HSP were autosomal dominant type I in 5 families and type II in 2, and autosomal recessive in 2 families.