A study is made of radiative transfer along a cylindrical duct using the one-dimensional approximation of Larsen. This method enables three-dimensional effects to be included in a one-dimensional equation. We have generalised Larsen's approach so that the internal reflection from the cylinder surface is governed by the Fresnel law. Two problems are considered. In the first, we assume an infinite cylinder with a plane source at the origin and, in the second, we have a semi-infinite cylinder with an incident beam on the surface. A Fourier transform is employed for the infinite cylinder and the Wiener-Hopf technique for the semi-infinite one. We are able to calculate the radiation intensity as a function of position along the cylinder and the mean square distance of travel of a photon. For the semi-infinite case, we obtain the albedo, the surface intensity and the mean distance of travel. Several features arise which demonstrate that the one-dimensional approximation preserves important features of the three-dimensional problem, thereby making the present approach a valuable tool in dealing with more complex problems.