Cellular characteristics of bronchoalveolar lavage (BAL) were investigated in 18 consecutive patients with Crohn's disease, who were free of clinical pulmonary symptoms and had normal findings on chest roentgenograms. Total BAL cell count and cellular viability of alveolar macrophages did not differ significantly between patients and control subjects. Percentage of alveolar lymphocytes was increased in 11 of 18 patients (from 18 percent to 79 percent). There was no apparent correlation between BAL differential cell count and abnormal pulmonary function tests noted in 11 patients, drug treatment or Crohn's disease site, and activity. These results demonstrate a high proportion of latent lymphocyte alveolitis as assessed by BAL, suggesting a latent involvement of the lung in Crohn's disease.