Failure of weaning from mechanical ventilation is thought to result from an imbalance between respiratory muscle capacity and respiratory demand. The ratio of respiratory rate to tidal volume (f/VT, rapid shallow breathing index) during spontaneous unsupported respiration increases when this imbalance exists, and may predict the success or failure of weaning from mechanical ventilation. Using f/VT, Yang and Tobin demonstrated a positive predictive value (PPV) of 0.78 (f/VT < or = 105 and weaning success) (1). To define the etiology of the 20% false-positive rate (FPR, f/VT < or = 105 and weaning failure), 94 patients who had an f/VT determined prior to extubation were studied prospectively. Of 84 patients with an f/VT < 100, 14 required reintubation within 72 h of extubation (FPR = 0.17, PPV = 0.83). Extubation in 13 of these 14 cases failed because of congestive heart failure, upper airway obstruction, aspiration, encephalopathy, or the development of a new pulmonary process. Only one patient needed reintubation solely because of the original respiratory process. Of 10 patients extubated with an f/VT > or = 100, four required reintubation, all because of the underlying respiratory process. This study confirms the high PPV for an f/VT < 100. The FPR of approximately 0.20 is best explained by extubation failure caused by processes for which f/VT is physiologically or temporally unlikely to predict success or failure. The negative predictive value (f/VT > or = 100 but extubation success) for f/VT may be lower than previously reported.