OBJECTIVE: Bronchoalveolar lavage (BAL) is a well established technique for the detection of pathogens in immunosuppressed children, but its diagnostic yield is variable. The aim of this study was to investigate whether BAL differential cell counts are helpful in the evaluation of pulmonary infiltrates in immunocompromised children. STUDY DESIGN: BAL was performed 28 times in 27 febrile immunocompromised children with pulmonary infiltrates. All patients were pretreated with broad spectrum antibiotics; 11 children also received amphotericin B. BAL was conducted with a flexible bronchoscope wedged in the area of maximal pathology as suggested by the chest radiograph or in the middle lobe in patients with diffuse interstitial radiographic changes. Differential cell counts were performed from cell smears obtained after centrifugation of BAL fluid. RESULTS: Bacterial or fungal organisms were detected in BAL fluid of 12 patients. Patients with bacterial or fungal infections (group 1) had a significantly higher percentage of granulocytes in BAL fluid both compared with patients with sterile BAL cultures (group 2) and with a control group of children without pulmonary disease (p < 0.001, Wilcoxon test). The proportion of lymphocytes was not different from the control group in group 1 but significantly increased in group 2 (p < 0.001, Wilcoxon test). Blood differential cell counts were not different in the two patient groups. Lymphocyte subsets of BAL fluid obtained in a subgroup of patients were not significantly different from controls. CONCLUSION: These data suggest that BAL differential cell counts may be a useful adjunct in the differential diagnosis of pulmonary infection in immunocompromised children.