This paper describes a genetic study of the chronic spinal muscular atrophies of late infancy and early childhood in Algeria. There were 50 index patients occurring in 44 kindreds and fourteen secondary cases. Genetic and nosological studies indicated that 52% of the patients constitute a genetically homogeneous subgroup with an age of onset between 3 and 24 months and an autosomal recessive mode of transmission. They also indicated that a large subgroup of index patients (48%) had a late age of onset, between 3 and 14 years. Such a large number of late presenting cases has not been reported in previous series. The majority of these cases are probably due to an autosomal recessive gene. A small proportion may represent new dominant mutations or nongenetic phenocopies. A possible sex influence on disease manifestations is discussed. A trend toward later male onset has been noted, and the degree of disability is more marked in males at or after the age of 10. Finally, some empirical risks for use in genetic counselling are presented.