BACKGROUND AND OBJECTIVE: A cylindrical light diffuser has been developed mainly for medical applications, including photodynamic therapy (PDT), in particular interstitial PDT, PDT of the bronchi, or intravascular PDT. STUDY DESIGN/MATERIALS AND METHODS: The diffuser is based on a polymer optical fiber. The coupling of the light out of the core is controlled by the roughness mechanically induced on the surface of the core. The light is then isotropically diffused by a thin layer of a scattering medium. The active length can be 100 mm or more, whereas the outer diameter is 1 mm or less. The diffuser is flexible and can be introduced in tissue though a hypodermic needle. RESULTS: The main property of this light diffuser is the homogeneity of the light intensity emitted along its whole active length and around its circumference (360 degrees). Various intensity profiles can be made, including M-shape profiles for a homogeneous irradiance (+/- 10%) at a certain depth in the tissue. Furthermore, the diffuser is essentially isotropic and its optical properties are hardly dependent on wavelength. CONCLUSIONS: A diffuser for interstitial and intraluminal PDT has been developed. It is flexible, homogeneous, independent on wavelength, and can be made with a very high length-to-diameter ratio.