Instability of the centromeric region of chromosome 1, and multibranched configurations formed by the short and long arms were seen in a brother and sister with facial dysmorphism, mental retardation and recurrent infections. No chromosomal abnormalities were seen in the parents, who were first cousins. The fragility of chromosome 1 was identified in amniotic fluid cells of the sister. A combined immunodeficiency characterized by a lack of immunoglobulin production, low numbers of T cells and a lack of cells with NK cell markers was diagnosed. This is the first report of familial occurrence of this unique chromosomal aberration. The cause may be an autosomal recessive gene defect.