Measles virus (MV) is the causative agent of subacute sclerosing panencephalitis (SSPE). We previously reported that the F gene of the SSPE Osaka-2 strain is the major determinant of MV neurovirulence. Because the sites and extents of mutations differ among SSPE strains, it is necessary to determine the mutations responsible for the SSPE-specific phenotypes of individual viral strain. In this study, recombinant viruses containing the envelope-associated genes from the SSPE Osaka-1 strain were generated in the IC323 wild-type MV background. Hamsters inoculated with MV containing the H gene of the Osaka-1 strain displayed hyperactivity and seizures, but usually recovered and survived. Hamsters inoculated with MV containing the F gene of the Osaka-1 strain displayed severe neurologic signs and died. Amino acid substitutions in the heptad repeat A and C regions of the F protein, including a methionine-to-valine substitution at amino acid 94, play major roles in neurovirulence.