We studied 13 patients with acute attacks of asthma to test the hypothesis that magnitude of dyspnea at rest correlates well with spirometry and with breath-holding time. Dyspnea ("falta de aire" in Spanish) was quantitated with a linear numerical scale from 0 to 10. We measured breath-holding time, breathing frequency (f), and FEV1 and FVC both expressed as percent of normal. Measurements were done on the patient's arrival and were repeated 3 to 6 times until dyspnea at rest disappeared or was minimal. Dyspnea magnitude, f, and breath-holding time correlated well with FEV1% and FVC% (r between 0.65 and 0.78), and better with changes in FEV1% and FVC% with respect to initial values (delta FEV1 y delta FVC; r between 0.80 and 0.89). Breath-holding time and f changed in proportion to magnitude of dyspnea (r = -0.85 and 0.87 respectively). Regression equations were: dyspnea = 6.34 -0.16 (delta FEV1) r = 0.80, and dyspnea = 7.82-0.105 (FEV1%) r = 0.62. Using multiple regression we improved prediction of FEV1% with easily obtained variables (R = 0.76). These results suggest that: 1) magnitude of dysnea, f, and breath-holding time correlate with severity of airflow obstruction in acute asthma attacks associated with dyspnea at rest; and 2) breath-holding time varies inversely with dyspnea magnitude when it is present at rest.