A descriptive epidemiological survey of hereditary ataxias and spastic paraplegias was conducted in the province of Torino, Italy (2,327 996 inhabitants). On prevalence day (31 December 1982) 142 patients were alive. Total prevalence was 6.1 cases/100,000 inhabitants (95% confidence limits = 5.1-7.0). The prevalence rate was 2.6 for recessive or sporadic juvenile ataxias, 1.3 for spastic paraplegias, 1.2 for autosomal dominant cerebellar ataxias, and 1.1 for late onset cerebellar ataxias. The prevalence of hereditary ataxias is similar in our province to that recorded in other populations, but hereditary spastic paraplegias are less frequent than in other populations.