We analyzed nucleotide changes in 3 genes, ARIX, PHOX2B, and KIF21A, in 6 patients of 3 families with congenital superior oblique muscle palsy. Three exons of ARIX, 3 exons of PHOX2B, and exons 8, 20, and 21 of KIF21A were amplified by polymerase chain reaction from genomic DNA isolated from the peripheral blood. The DNA fragments were directly sequenced in both directions. In 2 different families, a heterozygous nucleotide change, ARIX 153G>A, in the 5'.-untranslated region was found in common between a father and daughter with muscle palsy and between a mother and daughter with muscle palsy (Family No. 1 and No. 3). In the other family (Family No. 2), a heterozygous 15-nucleotide deletion, PHOX2B 1124del15, resulting in loss of 5 alanine residues in the alanine repeat of the protein, was found in the daughter with muscle palsy and her father with normal traits, but was not found in the mother with muscle palsy. No KIF21A nucleotide change was found in any patients. The ARIX 153G>A polymorphism might be a genetic risk factor for the development of congenital superior oblique muscle palsy.