Syndactyly type I is an autosomal dominant condition with complete or partial webbing between the third and fourth fingers or the second and third toes or both. We report here a previously undescribed phenotype of severe mesoaxial syndactyly and synostosis in patients born to affected parents. The characteristic features of these severe cases are (1) complete syndactyly and synostosis of the third and fourth fingers; (2) severe bone reduction in the proximal phalanges of the same fingers; (3) hypoplasia of the thumbs and halluces; (4) aplasia/hypoplasia of the middle phalanges of the second and fifth fingers; and (5) complete or partial soft tissue syndactyly of the toes. We report on three offspring with this phenotype from two different branches of a syndactyly type I family, suggesting that they may be homozygous for this condition. SSCP and linkage analysis indicated that neither HOXD13 nor other relevant genes in the chromosome 2q31 region was responsible for this phenotype.