The frequency of malformations of the kidney and urinary tract is much higher in patients with chromosome aberrations than in the general population. Sixty to 100% of "cat-eye" syndrome, 60 to 80% of Turner, 75% of trisomy 8, 33 to 70% of trisomy 18, 50 to 60% of trisomy 13, and over 50% of triploidy and tetraploidy patients may have such abnormalities. Renal and urinary tract malformations should be looked for in all patients with chromosome aberrations. Moreover, a chromosome study is indicated in any fetus with an ultrasonographic evidence of urinary tract abnormality.