Pulmonary histiocytosis X is characterized by an accumulation of CD-1-positive histiocytosis X cells in the lung, which also can be found in the bronchoalveolar lavage fluid (BALF). However, it has recently been demonstrated that CD-1-positive cells can also be detected in BALF of patients with other interstitial lung diseases and in healthy smokers. We therefore examined the frequency of CD-1-positive cells in a pool of patients with different pulmonary disorders, according to their smoking habits and diagnoses. We have studied the bronchoalveolar lavage in patients with pulmonary histiocytosis X (n = 6), sarcoidosis (n = 88), and in 97 patients with other miscellaneous lung disorders by using the immunoperoxidase method to detect CD-1-positive cells on glass slides. All patients with histologically proven histiocytosis X displayed more than 5% CD-1-positive cells, whereas patients with other pulmonary disorders showed no more than 3.6% CD-1-positive BAL cells. The dividing line of 5% CD-1-positive cells was not influenced by patients' smoking habits. The identification of CD-1-positive cells in BALF appears to be useful in diagnosing pulmonary histiocytosis X.