Several recently proposed medical procedures involve intralumenal laser irradiation at power densities below the vaporization threshold. Typically the radiation is transported to the tissue to be treated by an optical fiber, and then applied to the inner wall by an energy diffusing source. The tissue is heated but not removed; in fact extensive tissue vaporization is potentially disastrous. The goal is therefore to elevate the tissue to a defined temperature within a range bounded by the threshold for effectiveness and the threshold for damage. The temperature reached is a function of the dimensions and properties of the tissue and of wavelength, exposure duration, power density, and certain boundary conditions. For example, a heat sink can be placed in contact with the inner surface, substantially altering the temperature distribution. We have constructed a simple model to calculate the radial temperature distribution in a cylinder of tissue subjected to continuous or long pulse optical irradiation from an axial source. The intent is to permit dosimetry based on experimentally supported calculations, rather than on trial and error. The temperature reached results from two competing effects: absorption of the radiation and conductive processes. The wavelength dependence is introduced through choice of absorption and attenuation coefficients, with tissue properties summarized in the thermal diffusivity. The effect of scattering is assumed to be no more than an alteration of the attenuation coefficient; the possibility of relaxing this assumption is considered. The model allows investigation of the effects of power density, pulse duration, and heat sinking on the time evolution of the temperature distribution.