Hearing loss in patients with X-linked agammaglobulinemia is often attributed to recurrent infections. However, recent genetic studies suggest a different etiology in some patients. We present three unrelated patients, 6, 9, and 14 years of age, with large deletions of the terminal portion of the Bruton tyrosine kinase (Btk) gene extending 4.2–19 kb beyond the 3′ end of the gene. The DNA immediately downstream of the 3′ end of Btk contains the deafness-dystonia protein gene (DDP). Mutations in this gene have recently been shown to underlie the Mohr–Tranebjaerg syndrome, which is characterized by sensorineural deafness, dystonia, and mental deficiency. Besides the immunodeficiency, our patients exhibited progressive sensorineural deafness. The clue to an associated hearing problem was delayed development of speech in one patient and post-lingual deafness noticed between the age of 3–4 years in the other two. These patients have not yet exhibited significant associated neurologic deficits.