The ulnar and posterior tibial conduction velocities were measured in 29 children with spinal muscular atrophy, 14 of whom had the servere form of the disease. The ulnar nerve velocity was slow in 12 of the 14 severely affected infants, but normal or fast in 11 of 14 children less severely affected. The corresponding results for the posterior tibial nerve were slow velocities in 11 of 12 infants in the severe group and normal or fast in all 11 infants less severely affected. The difficulty in distinguishing infantile spinal muscular atrophy from peripheral neuropathy is emphasized.