The light distribution within a treatment volume is determined by the source geometry (e.g. superficial or interstitial illumination) and the optical interaction coefficients of the irradiated tissue. We have measured the energy fluence rate at various points within tumours undergoing irradiation with 630 nm light for photodynamic therapy for several source geometries. The relative positions of source and detector fibres were determined using CT scanning techniques. The results of the measurements were then applied to solutions of the diffusion theory which allowed the determination of the absorption coefficient (sigma a = 30.5 +/- 16 m-1), the reduced scattering coefficient (sigma' s = 941 +/- 735 m-1), the effective attenuation coefficient (sigma eff = 261 +/- 49 m-1) and the build-up coefficient which relates surface irradiance to the energy fluence rate at depth (k = 1.6 +/- 0.6). Knowledge of these coefficients allows the transmission of light through tissue to be predicted and hence the optical dosimetry of subsequent treatments to be planned more effectively.