In a multicenter, city-wide study of the use of bronchoalveolar lavage for the evaluation of diffuse interstitial lung diseases, the occurrence of specimens unsuitable for analysis was evaluated. Using a standardized bronchoscopy technique, 26 physicians obtained 1,588 lavage specimens from 787 patients over a 52-month period. After transport to and processing in one laboratory using standardized procedures, all specimens were interpreted by one pathologist. Specimens were considered unsatisfactory if they contained: (1) a paucity of alveolar macrophages (i.e., less than ten alveolar macrophages/high-power field), (2) excessive numbers of airway-derived cells (i.e., more than the alveolar macrophages present), (3) a mucopurulent exudate, (4) cells altered by degeneration or (5) laboratory artifacts. Using these criteria, 30.4% of the specimens were considered unsuitable for analysis. There were no significant differences in the frequency of unsatisfactory specimens among participating physicians and institutions or between smoking and nonsmoking patients. Appraisal of alveolar inflammatory and immune effector cells in bronchoalveolar lavage specimens from patients with interstitial lung disease should include an assessment for contamination from airways proximal to the terminal bronchioles before conclusions are drawn about the activity of alveolar inflammation.