X-linked, or Bruton's, agammaglobulinemia (XLA) was described in 1952 as the congenital inability to form antibodies. Patients were typically infants or young children with recurrent, severe bacterial infections. Other, milder cases of hypogammaglobulinemia were considered "acquired," and often presented later in life. Since the discovery of the defective gene in XLA in 1993, it has been shown that a significant number of male patients with sporadic or acquired hypogammaglobulinemia actually have XLA. We present here a case of atypical XLA and discuss similar cases in the literature. We conclude that any male with hypogammaglobulinemia, regardless of age of presentation, might have XLA. Males with low B-cell numbers are particularly likely to have XLA and should have Bruton's tyrosine kinase levels assessed.