We carried out a retrospective analysis of 117 patients with severe combined immunodeficiency who were examined in a single center between Jan. 1, 1970, and Jan. 1, 1992, for the purpose of evaluating disease onset, progression, and outcome. The frequency of case referral increased from 8 from 1970 to 1975 to 56 from 1986 to 1991. The most frequent phenotype was T-/B+ (absence of T lymphocytes and presence of B lymphocytes) (n = 51); there were 36 cases of alymphocytosis, 16 of adenosine deaminase deficiency, 13 of Omenn syndrome, and 1 of reticular dysgenesis. Protracted diarrhea and lung infections were the main infectious complications; infection with bacillus Calmette-Guérin occurred in 10 of 28 vaccinated patients, but none of the six recipients of oral polio vaccine subsequently had poliomyelitis. The presence of maternal T cells was suspected or proved in half the patients with alymphocytosis or T-B+ severe combined immunodeficiency but did not occur in the other forms of the disease. Of the 117 patients, 22 died before transplantation could be performed. Adenosine deaminase deficiency and Omenn syndrome were more frequently associated with death before hematopoietic stem cell transplantation was possible. Fetal liver transplantation was successful in 1 of 10 cases. The survival rate among the 30 recipients of bone marrow with identical human leukocyte antigens (HLA) was 80%, with a median follow-up of 129 months; 23 of 25 patients recovered full immune function. The survival rate among the 50 recipients of HLA-haploidentical T cell-depleted bone marrow was 56%, with a mean follow-up of 35 months. Of the latter patients, 10 (35%) still require immunoglobulin substitution. There has been a trend toward improvement in the survival rate of haploidentical bone marrow recipients, presumably because of more effective infection-control measures and better transplantation strategy.