The performance of a Tm :YVO4 laser has been optimized by varying the thickness of the laser crystal and the reflectivity of the output coupler. This has led to the measurement of a slope efficiency of 48% for the laser output. Formulas describing the saturation of the pump and the upper laser level in the presence of up-conversion loss have been developed. Fitting of the experimental data to the saturation curves calculated using these formulas has yielded an up-conversion rate constant from F-3(4) to H-3(5) of 2 . 10(-18) cm3 s-1.