Method for treating chronic rhinosinusitis in a human patient. In the method a photosensitizing agent such as indocyanine green can be applied to color a nasal or sinal treatment site harboring infectious microorganisms. The photosensitizing agent can have an energy absorption peak in a near infrared wavelength region of the electromagnetic spectrum. The method can also include applying light energy from a laser source at a near infrared wavelength or wavelengths to the colored treatment site, for a duration sufficient to control the microorganisms. Apparatus for performing the method is also disclosed.