Tests were carried out on the usefulness of doubling the reference resistor during airways resistance measurement by forced oscillation method (set frequency technique). Measurements on 110 random subjects using the standard reference tube (Zo = 10 hPa/1/s) for comparison showed almost identical Ros values using the experimental tube, while the latter gave higher Ros values as obstruction increased. Comparisons with the bodyplethysmographic Rt on 65 patients with a chronic obstructive airways disease yielded a closer approximation to the identity curve from 7.0 hPa/1/s using this modified technique. The practical conclusion is to recommend the use of the double reference resistor for the basic clinical diagnosis of obstructive airways disease, both in quantifying advanced grade obstructions and in assessing the damaged respiratory mechanism in chronically ill patients with pronounced ventilatory resistance.