Human tumor xenografts in immune-deficient animals are used to establish tumor growth curves and for studying the effect of experimental therapy on tumor growth. In this review we describe a method for making serial measurements of tumor size in the nude mouse model as well as methods used to transform the experimental data into useful growth curves. A transformed Gompertz function is used as the basis for calculating relevant parameters pertaining to tumor growth and response to therapy. The calculations are facilitated by use of a computer program which performs the necessary calculations and presents the growth data in graphic form.