An 8-year-old girl developed ataxia-telangiectasia. Western blotting of lysate revealed absence of the ATM protein, and 2 mutations in the ATM gene were found. Subsequently, the patient developed increased respiratory symptoms. Open lung biopsy revealed lymphocytic interstitial pneumonitis, which is not characteristic of ataxia-telangiectasia. There was a therapeutic response to glucocorticosteroid treatment.