BACKGROUND AND OBJECTIVE: A technique to manufacture a stable, reproducible three-dimensional optical phantom is presented. This phantom reproduces the tissue's optical properties as well as the geometry and, to some extent, the mechanical properties of the organ concerned. Easy to make and to handle, this phantom is a useful tool for numerous medical applications involving light interaction with biological tissues. STUDY DESIGN/MATERIALS AND METHODS: The phantom is based on a transparent two-component silicone, which is molded into the desired shape and cured at room temperature. Specific optical properties are obtained by adding scatterers (Al2O3 particles or polystyrene microspheres) and absorbers (dyes or pigments). A method to measure the radiant energy fluence rate in the phantom is described. This method is based on a small isotropic optical detector. RESULTS: A three-dimensional phantom of the bronchial tree is presented. This phantom is used for testing new light distributors designed for photodynamic therapy of the bronchi. CONCLUSION: The proposed technique allows one to produce a stable three-dimensional phantom with accurately predictable optical properties.