Clinical and laboratory findings in two girls with a disorder phenotypically indistinguishable from typical X-linked agammaglobulinemia (XLA) are described. To examine the possibility that subtle defects in the X chromosome might explain the findings, detailed genetic studies were performed on one of these patients. Cytogenetic studies showed a normal 46XX karyotype. Southern blot analysis of her DNA showed that she had inherited a maternal and a paternal allele at sites flanking the locus for typical XLA at Xq22, making a microdeletion or uniparental disomy unlikely. To determine whether both of her X chromosomes could function as the active X, somatic-cell hybrids that selectively retained the active X were produced from her activated T cells. A normal random pattern of X inactivation was seen. Of 21 T-cell hybrids, 3 retained both X chromosomes, 7 had one X as the active X, and 11 had the other X as the active X. We have interpreted these studies as indicating that there is an autosomal recessive disorder that is phenotypically identical to XLA.