Severe combined immunodeficiency (SCID) is a rare syndrome of profoundly impaired immunity, most often X-linked (XSCID). In past generations male infants with XSCID succumbed to infections during the first year of life, but prompt diagnosis and bone marrow transplantation currently make survival possible for over 80%. This treatment typically requires hospitalization for several months; thus, the burden on the family is considerable. We assessed the psychological impact on sibs of boys with XSCID. Forty adult sibs from families studied by J.M.P. were interviewed by J.H.F., and rating scales developed. The majority expressed distress over prolonged maternal absence during the affected child's hospitalization; 67% believed the mother had unsuccessfully mourned son(s) who died of XSCID. Half of the sibs reported that communication in the family about XSCID had been poor. Families with a spontaneous mutation were significantly more likely to report separation issues (P = 0.05), perhaps due to stronger maternal guilt. Family communication was significantly related to parental mourning (P = 0.001) and to survivor guilt (P = 0.05). Difficulties for daughters included desire to repair the mother's loss of her own child, as well as attempts to undo feelings of being flawed, by heightened wishes to bear a healthy son. In light of these findings we suggest: 1) bone marrow transplantation and the period of isolation places great stress on the family; parents need help balancing needs of well sibs with needs of the affected son; 2) parents need help with mourning the loss of a son so family secrets will not prevail; and 3) sibs, both bone marrow donors and non-donors, face psychological risks and need support.