The rise time of epicardial temperatures on the left ventricular free wall during endocardial transcatbeter delivery of Nd:YAG laser energy, recorded by thermography has sparked interest in anomalous optical phenomena. Transmission measurements on whole blocks of myocardium showed a decrease in the attenuation coefficient at high power densities. The definition of the attenuation coefficient is the natural log of the ratio of the incident and the transmitted beams, divided by the physical thickness of the sample. When Nd:YAG laser light at the 1064 um wavelength irradiated fresh myocardial tissue from the free left ventricular wall, the incident power density increased from 0.0026 W/mm2 to approximately 200 W/mm2, the attenuation coefficient dropped from 0.97 ± 0.11 mm−1 to 0.77 ± 0.07 mm−1. This would indicate a non-linearity in the optical properties of the tissue as a function of the laser irradiation power density.