The ataxia-telangiectasia gene (designated ATM) has been identified by positional cloning. Retrospective studies in cystic fibrosis (CF) have shown that the illness and death of a sib have far-reaching ramifications on the surviving sib's life. However, there have been no studies on sibs of children and adults with ataxia-telangiectasia (A-T). Thirty-five sibs from 24 families, including 26 adults and 9 adolescents, were drawn from the University of California, Los Angeles; the A-T Clinical Center at the Johns Hopkins University School of Medicine; and the A-T Children's Project Interviews indicated that, unlike CF, the visibility of A-T caused less resentment of time or attention given to the affected, less guilt, and less identification or idealization of the affected, but more embarrassment and shame. It may be said that, unlike CF, the dynamic is one of burden rather than trauma. The specific phenotypic expression of a genetic illness predisposes the way in which the well sib will encounter problems. In A-T affected families, one must help families locate caregiving resources so they will not burden their well children; sibs need help with managing feelings of embarrassment and shame. Geneticists must be alert to understanding characteristic differences of disorders they encounter in order to offer the most appropriate support to affected families.