The examination of the chromosomes of a seven year old child with severe mental retardation, allowed us to bring to light, beside a majority of cells with a normal karyotype, a population of about 10% of cells in which chromosome 7 showed a gap in the juxtacentromeric area of its short arm. The degree of this alteration varied from the simple angulation to the complete break with short arm's loss. The functional and structural signification of this abnormality is discussed and this case is compared with previous observations concerning the abnormalities of this area of chromosome 7 (specially the causal aberrations occuring in cell cultures).