To investigate the number of cells to be counted in cytocentrifuged bronchoalveolar lavage (BAL) fluid preparations in order to reach a reliable enumeration of each cell type.A total of 136 BAL fluid samples for patients with suspected pneumonia or interstitial lung disease were investigated. Differential cell counts were performed on May-Grünwald-Giemsa-stained cytocentrifuged preparations by 2 observers, each differentiating 500 cells. Reliability for the enumeration of each cell type was expressed as phi value, as calculated in generalizability theory.For polymorphonuclear neutrophils (PMNs), alveolar macrophages, lymphocytes and eosinophils, an acceptable phi value of > or = .95 was reached at a count of 300 cells by 1 observer. Mast cells reached a phi value of only .674 at a count of 500 cells by 1 observer, precluding a reliable count. At a count of 500 cells by 1 observer, squamous epithelial cells, bronchial epithelial cells and plasma cells displayed phi values of .868, .903 and .816, respectively.At a count of 300 cells, PMNs, alveolar macrophages, lymphocytes and eosinophils are reliably enumerated in cytocentrifuged BAL fluid samples.