We compared the usefulness of several spirometric measurements in detecting asthmatic reactions after allergen bronchial challenge in fourteen asthmatic children. All of the children had a history suggesting mite-induced asthma and eleven had a 3 mm or larger diameter weal on prick testing with Dermatophagoides farinae extract. On bronchial challenge testing with dilutions of this extract twelve children had an early asthmatic reaction and nine had a late asthmatic reaction. In decreasing order of sensitivity for detecting the early asthmatic reaction the tests ranked as follows: FEV1, FEF50%, FEF25-75%, PEFR and FVC. For the late asthmatic reaction the order was FEF50% FEV1, FEF25-75%, PEFR and FVC. No single test identified all the early or all the late reactions but the FEV1, a test useful for indicating large airways obstruction, when combined with the FEF25-75%, a test influenced by small airways obstruction, detected all early and late asthmatic reactions. The FEF50%, was a sensitive test but was the only one to become falsely positive. It became falsely positive in four patients. Although the FEV1, was the most useful single test the results suggest that it is, by itself, an inadequate indicator of the asthmatic reaction and that it should be used with the FEF25-75%, to ensure the detection of all asthmatic reactions induced by allergen bronchial challenge testing.