Coupling to spurious modes at imperfections in a long circular waveguide may cause the TE01loss to increase and to fluctuate randomly with frequency. The statistical properties of the TE01loss/frequency curve can be expressed in terms of the statistical properties of the random geometric imperfections of the guide and therefore in terms of its mechanical tolerances. Such imperfections as poor joints and elliptical waveguide have negligible effect under reasonable tolerances. However, random deviations of the guide axis from a straight line may have very serious effects under certain conditions. The use of mode filters or of helix waveguide to increase the spurious-mode loss reduces the TE01loss fluctuations but leaves the average TE01loss due to mode conversion relatively unaffected.Experimental measurements of loss/frequency curves, of spurious-mode content and of mechanical tolerances which have been made on existing waveguides support these conclusions. Random straightness deviations introduced during manufacture of the waveguide are the primary cause of the observed transmission behaviour.