Spinocerebellar ataxia type 7 (SCA7) is an autosomal dominant cerebellar ataxia caused by CAG repeat expansion. We found expansion at SCA7 locus in only two out of 235 Indian families clinically diagnosed for ataxia. In one of the families, a de novo mutation was observed wherein a paternal allele in intermediate range of 31 CAG repeats expanded to 59 in the offspring leading to the disease. No expanded alleles were observed in the sperm of the transmitting parent by small pool PCR. This suggests that de novo expansion by a pre-zygotic event is unlikely and could be post-zygotic. SCA7 expanded alleles from the two families were present on different genetic backgrounds, indicating multiple origins of the mutation. © The Japan Society of Human Genetics and Springer-Verlag 2005.