Bedside measurement of respiratory frequency is commonly performed in a cursory manner and judged to be of little clinical importance. However, in a recent study of patients being weaned from mechanical ventilation, we found that tachypnea was quite accurate in predicting an unsuccessful weaning outcome. The present study was undertaken to examine the relationship between nonobtrusive measurements of respiratory frequency, using a calibrated inductive plethysmograph, and detailed measurements of pulmonary function in 11 adult patients with cystic fibrosis of varying severity. Respiratory frequency was increased in the patients with cystic fibrosis compared with a group of healthy control subjects, as was minute ventilation and mean inspiratory flow. Respiratory frequency was a sensitive predictor of respiratory dysfunction, being significantly (p less than 0.05) correlated with airway obstruction (r = 0.76), hyperinflation (r = 0.52), arterial oxygenation (r = -0.59), rib cage-abdominal discoordination (r = 0.54), and maximum ventilation during exercise (r = 0.66). Despite the presence of tachypnea, the patients did not display shallow breathing; indeed, tidal volume was not correlated with any of the above abnormalities. In conclusion, respiratory rate was a useful indicator of respiratory dysfunction in this group of patients with cystic fibrosis.