A low concentration formulation for hydrophobic photosensitizers (PS) and improved method for photodynamic therapy ("PDT"). It was found that PDT treatments using the disclosed low concentration formulations provide for more accurate, more efficient and more convenient dosing. It was found that the inventive formulation (1) reduces the time for a therapeutically effective level of photosensitizer to accumulate in diseased tissue and (2) reduces the time for achieving a sufficient ratio of photosensitizer in diseased tissue vs. healthy tissue. As a result, the formulation of the invention reduces the time interval between PS application/administration and irradiation (the drug-light interval or "DLI") and can provide for a "same day" PDT treatment option. The inventive formulation can be used for PDT treatment regimes where photosensitizers are administered in at least one preselected dose, including a low concentration therapy for PDT. In particular, when meta Tetra-Hydroxy-Phenyl Chl orin (m-THPC) is the photosensitizer then a concentration of 0.8 mg/ml to 0.04 mg/ml in a mixture of pure propylene glycol and ethanol in a 3:2 volume ratio accumulates in diseased tissue and differentiates between diseased tissue and normal tissue sufficiently quickly for `one day' or overnight administration and activation treatment procedures to be possible.Una formulación de baja concentración para fotosensibilizadores hidrofóbicos (PS) y un método mejorado para terapia fotodinámica ("PDT"). Se encontró que los tratamientos con PDT usando las formulaciones de baja concentración descritas, proporcionan una dosificación más exacta, más eficiente y más práctica. Se encontró que la formulación de la invención (1) reduce el tiempo para lograr una proporción suficiente de fotosensibilizador en el tejido enfermo contra el tejido sano. Como resultado, la formulación de la invención reduce el intervalo de tiempo entre la aplicación/administración de PS y la irradiación (el intervalo de luz
