Two methods for measuring respiratory mechanics by forced oscillations were compared, one using a frequency spectrum of 2–32 Hz produced by a pseudorandom noise generator (PRN), the other (Siregnost FD 5) a constant frequency of 10 Hz. In normals and patients with various lung diseases real part of impedance, Re, and phase angle, ψ, determined at 10 Hz with the two devices correlated well; however, Re measured by Siregnost FD 5 was always higher when compared to PRN. In patients with airway obstruction frequency dependence of resistance and reactance could be shown to be related to the severity of the disease. This important information cannot be obtained by Siregnost FD 5. As Siregnost FD 5 is a simple and reliable method, it is a valuable completion of standard lung function methods. However, for detailed analysis the respiratory system should be investigated at a variety of frequencies.