The fluorescence properties of human tissue are known to be temperature dependent. The most apparent effect of this dependence is the inverse relationship between fluorescence and temperature. In this study, we used fluorescence and diffuse-reflectance spectroscopy to investigate the effects of temperature on fluorescence, thermal coagulation, and tissue optical properties. Human tissue from the breast and abdomen were examined in vitro, and human skin was examined in vivo using a fluorescence and diffuse-reflectance system to observe the effects of temperature on fluorescence and optical properties. Fluorescence measurements were carried out using a pulsed nitrogen laser at 337 nm for excitation and a thermal camera for temperature measurements. Thermal variation of the specimens was provided by a phosphate buffered saline solution for the in vitro experiments and an ice pack and heat lamp for the in vivo experiments. In vitro temperatures varied from 0°C to 70°C and in vivo temperatures varied from 15°C to 40°C. Optical property measurements and Monte Carlo simulations were carried out on the in vitro samples for different levels of thermal exposure. Results of both the in vivo and in vitro experiments indicate that optical properties of human tissue change at high temperatures, primarily due to increased scattering. In addition, certain internal processes may have contributed to a decrease in fluorescence with increasing temperature. Some of these effects were found to be reversible before a certain temperature threshold, while some effects of coagulation on fluorescence and optical properties were not reversible.