The hypothesis that a change in white blood cell (WBC) count might be a useful predictor of the onset of adult respiratory distress syndrome (ARDS) was tested by monitoring total and differential WBC count in 40 patients at high risk of ARDS. ARDS developed in 10 patients and in 8 of these the systemic leucocyte count fell to less than 4200 cells/microliter. The other 2 patients showed a fall from 25 400 and 15 000 cells/microliter to 4500 and 6000 cells/microliter, respectively. Only 4 of the 30 patients in whom ARDS did not develop showed a fall to less than 4200 cells/microliter. The acute fall in WBC that was associated with the onset of severe respiratory failure indicates that frequent measurement of the leucocyte count in high-risk patients may be useful in identifying those in whom ARDS will develop.