Measurement of forced expiratory volume in one second (FEV1) is commonly used in bronchial challenge testing in laboratory and epidemiological studies but has certain disadvantages. We have therefore studied the repeatability and validity of a derived measurement of respiratory resistance (Ros) obtained with the Siregnost FD5 impedance oscillometer (Siemens). Repeatability was estimated in 25 non-asthmatics and 28 asthmatics and compared with that of PEFR, FEV1 and specific conductance. PEFR and FEV1 were the most repeatable. Repeatability for measurements with the oscillometer as measured by the intraclass correlation coefficient for a single measurement was 0.75 for non-asthmatics, but was less good for asthmatics (intraclass correlation coefficient 0.56). To assess the validity of respiratory resistance measured during a histamine bronchial challenge test a sample of 20 hospital personnel was studied on four occasions, FEV1 and Ros both being measured twice. Ros changed at lower doses of histamine than FEV1, but the intraclass correlation coefficient for repeatability of change only reached 0.6 at an absolute dose of 3.41 mumol histamine. The estimated provocation dose of histamine producing a 35% fall in Ros was 8.70 mumol (95% range for a single measurement +/- 1.11 doubling doses), that producing a 10% fall in FEV1 was 8.32 mumol (95% range +/- 1.04 doubling doses) and a 20% fall in FEV1 11.48 mumol (95% range +/- 1.11 doubling doses). Measurements obtained with the Siregnost FD5 oscillometer are repeatable. The use of Ros during bronchial challenge testing is valid, but shows insufficient advantage over FEV1 to support its use in epidemiological studies.