A simple procedure is presented for determining the irradiance of light striking tissues during light delivery using a microlens optical fiber via an endoscope. The particular example is photodynamic therapy (PDT) of laryngeal cancer. A microlens optical fiber is an optical fiber with a small lens assembly at its terminus which magnifies the image of the end of the fiber yielding a uniform irradiance in a plane distant from the fiber. Prior to the therapy, the relation between fiber-target distance, h [cm], and the diameter of the uniform beam, d(h) [cm], was established and the area of the beam was calculated: A(h) equals (pi) d(h)2/4. The laser power P [W] required to achieve a desired irradiance E [W/cm2] was P(h) equals EA(h), and this relation was prepared as a simple graph for routine use in the clinic. During the therapy, the doctor advances the optical fiber through the working channel of the endoscope to touch the target tissue site while observing through the optical channel, and marks the fiber on the outside of the endoscope. The doctor then retracts the fiber until an aiming beam transmitted through the fiber fully illuminates the desired target area, and again marks the fiber on the outside of the endoscope. The difference in the two marks on the fiber outside the endoscope yields h, the height of the fiber above the target tissue. The laser operator then uses the P(h) equals EA(h) graph to select the proper laser power to achieve the desired E. Although trivially simple, this dosimetry procedure was critical to the proper implementation of PDT for laryngeal cancer.