Lymphocytes of female carriers of X-linked severe combined immunodeficiency (XSCID; McKusick 300400; HGM genetic locus designation SCIDX1) exhibit nonrandom X chromosome inactivation. This phenomenon reflects a tissue-specific selective disadvantage for lymphocyte progenitors with an XSCID mutation on the active X chromosome and presumably is analogous to the process that inhibits T-cell development in affected boys with a single XSCID-bearing X chromosome. We investigated the specificity of T-cell X chromosome inactivation pattern as an indicator of immunodeficiency carrier status, as follows: X-inactivation ratios determined in a control group of noncarrier women exhibited a wide range, 20%-86% of T-cells with the paternal X active. Maximum-likelihood analysis of these data suggested that, in humans, mature T-cells are derived from a small pool of only about 10 randomly inactivated stem cells. Despite the wide variability in normal X-inactivation ratios, X inactivation in XSCID carriers appeared far more markedly skewed. Therefore a maximum-likelihood odds-ratio test was developed and proved to be successful in predicting the carrier status of women in XSCID pedigrees. This test has made it possible to identify XSCID carriers among mothers of boys with the heterogeneous syndrome of sporadic severe combined immunodeficiency.