The case reported here of an infant who presented with Pneumocystis carinii pneumonia, CD4+ lymphopenia, and hypogammaglobulinemia attributable to severe combined immunodeficiency (SCID). This report discussed treatment of Mycobacterium bovis bacillus Calmette-Guèrin disease with unrelated cord blood transplantation in addition to antituberculous therapy, by adoptively transferring donor immunity with induction of mixed chimerism. Because of the unique nature of umbilical cord blood hematopoietic cells, engraftment without conditioning may be possible in SCID patients without fully matched donors.