We have defined the clinical presentation and course of X-linked agammaglobulinemia (X-LA) by means of a multi-center retrospective survey of 96 patients. Infections were the most common presenting feature of patients with X-LA. The most frequent infections involved the upper respiratory tract (75%), lower respiratory tract (65%), gastrointestinal tract (35%), skin (28%), and central nervous system (16%). Clinical clues to the diagnosis of X-LA were the chronic or recurrent nature of infections, a family history of immunodeficiency, and infections at more than one anatomic location. Infections remained a significant problem after the diagnosis of X-LA was made and gamma-globulin prophylaxis had been instituted. One or more chronic infectious diseases occurred in 71% of patients. The respiratory tract was the most common site of disease, and the gastrointestinal tract was relatively spared. Patients died at a mean age of 17 years. The two major causes of death were chronic pulmonary disease with resultant cardiac failure, and disseminated viral infections which characteristically caused a dermatomyositis-like syndrome, hepatitis, pneumonitis, and meningoencephalitis.