Tetrasomy of the short arm of chromosome 9 constitutes a rare condition resulting in a well clinically recognized syndrome. In our case, in addition to the characteristic phenotype at birth, the existence of a hernia-type Bochdalek diaphragmatic defect was found. Cytogenetic analysis revealed a nonmosaic case of an isochromosome of the entire short arm of chromosome 9 with no involvement of the heterochromatic region of the long arm: 47, XX, +i (9p). Because chromosome 9 contains several gene locus for enzymes and receptors of the retinoid pathway, this case potentially contributes to retinoid hypothesis in the etiology of congenital diaphragmatic hernia. D