In the lymphocytes of a girl with microcephalia, hypertelorism, antimongoloid palpebral slant, bilateral cataract, severe dental caries, systolic murmur, back-rotation of kidneys and severe psychomotoric retardation, a ring chromosome G was found in all cells scored. Using the Giemsa banding technique it was possible to identify the ring chromosome as number 21. Similarity with previously published cases of partial G monosomy is discussed.