Thirteen patients with lymphocytic interstitial pneumonitis were seen at the Mayo Clinic from 1966 through 1976. The group included nine women and four men, with a mean age of 50.7 years. Their primary complaints were cough, dyspnea, and loss of weight. Chest roentgenographic features were predominantly basilar, coarse interstitial-alveolar infiltrations. Pulmonary function studies showed restrictive ventilatory impairments with a low CO diffusing capacity in all 13 patients. Lung biopsies in all cases showed diffuse interstitial infiltrations, consisting of mature lymphocytes and plasma cells. Ten of the 13 patients had an associated dysproteinemia, including two patients with hypogammaglobulinemia. Three patients had coexistent Sjögren's syndrome, including two with localized amyloidosis. Although lymphocytic interstitial pneumonitis is a histologically distinct pulmonary lesions, it occurs with a variety of immune disorders.