We report a 5 1/2-year-old girl whose clinical symptoms are consistent with diagnosis of the cat-eye syndrome. The prominent symptoms are: anal stenosis, preauricular tags and pits, coloboma of the iris, doubling of the pelvis and ureter on both sides, vesicourethral reflux on the right side and normal mental development. Leucocyte alkaline phosphatase is normal. Chromosomal analysis shows a supernumerary submetacentric chromosome. This extra chromosome is smaller than the G-group chromosomes and has satellites on the short and long arms. Autoradiography after 3H-thymidine incorporation shows a late-labeling marker chromosome. After using the Giemsa-banding technique, the chromatides demonstrate dark bandings with only soft, unstained satellites. With the fluorescence method, one can see spotlike fluorescence of the satellites on both arms and diffuse fluorescence of the hetero-chromatic segments. In addition, the C-bandings demonstrate a homogeneous dark staining of the chromatids, but we did not find stained satellites. Using the Giemsa-11 technique one can see the 47th chromosome with predominantly heterochromatic parts, but small euchromatic segments are visible between them. Satellites are unstained. Using currently accepted cytogenetical methods, it is not possible to identify the origin of this supernumerary marker chromosome.