A four-month-old girl with facial dysmorphism, moderate mental retardation, immune deficiency (decreased IgG and IgA and absence of IgM), centromeric instability of chromosomes 1, 9, 16 and very rarely of chromosome 2, and disposition to formation of multibranched chromosomal figures, is described. The case is the fifth described with such chromosomal and immune abnormalities, which prove the existence of a new syndrome. The authors suggest an autosomal recessive inheritance.