An analysis of segregation and sex ratios, and of sex influence, was undertaken in a series of 78 index patients with acute infantile spinal muscular atrophy (SMA type I). The sex ratio of index patients was 2.0, and the excess of males was shown to occur principally among sporadic cases. The sex ratio of familial cases did not differ significantly from 1.0. The implications of this are discussed. No sex influence on age at onset, or on life expectancy, was present. The segregation ratio (Weinberg Proband method) was 0.29 for all index cases, and 0.26 for all cases excluding those referred specifically to a genetic counselling clinic. Autosomal recessivity is confirmed for this disease, with the probable inclusion of unrecognised male phenocopies in clinical series.