Bronchoalveolar lavage (BAL) has come into widespread use as a tool to diagnose and manage various lung diseases. However, the usefulness of BAL is based on the assumption that the cells recovered in BAL fluid accurately reflect cellular populations in the lung parenchyma. To test this hypothesis, random source (n = 16) dogs were given intravenous phorbol myristate acetate (PMA) to produce acute, diffuse lung inflammation. Dogs were divided into three groups. Group I animals (n = 2) underwent BAL and open lung biopsy at time zero. Group II (n = 7) animals underwent hourly BAL after PMA with open lung biopsy at 3 h. Group III (n = 7) animals underwent hourly BAL after PMA with open lung biopsy at 6 h. When BAL cell populations were compared with the corresponding biopsies, there was a direct correlation (r = 0.67) between BAL neutrophil percentages and neutrophils present in histologic sections. These findings suggest BAL accurately reflects changes in the lung parenchyma in acute lung disease.