Nine cases are described of a demyelinating peripheral neuropathy that had an onset in infancy. The clinical features conformed to those of type III hereditary motor and sensory neuropathy or Dejerine-Sottas disease. All showed a severe neurological deficit and had profoundly reduced nerve conduction velocities. Amongst these cases we identified four novel point mutations in the peripheral myelin protein 22 (PMP22) gene. These were Ser72Trp, Ser76lle and Leu80Pro. The Ser72Trp mutation was dominantly inherited by a mother and son, both severely affected. Two novel mutations in the gene for P0 myelin protein were also detected. These were Ile134Thr in exon 3, and a complex rearrangement in exon 4. The remaining three patients had presumed autosomal recessive inheritance. In these, no abnormality for the PMP22 and P0 genes was detected and a mutation at another locus or loci seems probable. On nerve biopsy the final two cases were shown to be examples of hereditary neuropathy with focally folded myelin sheaths. One showed both bulbar and diaphragmatic involvement. It is concluded that hereditary demyelinating neuropathy of infancy is genetically heterogeneous. Mutational screening for the PMP22 and P0 genes and nerve biopsy are therefore merited in patients with a childhood demyelinating neuropathy that is more severe than usual and in whom a chromosome 17 duplication is not present.