Tumor growth inhibition data in in vivo anticancer experiments are commonly analyzed using the treatment-to-control ratio (TCR). Parametric and nonparametric confidence interval approaches for this ratio are introduced, enabling a quantitative statistical decision. The growth curves are characterized by the area-under-the-curve technique, adjusted for animal-specific survival. Simple simultaneous approaches are proposed for complex designs, including several treatment or dose groups. This implementation makes decision making easier for the pharmacologists through the use of simple diagrams for the treatment-to-control ratios and their confidence intervals. Tumor inhibition and regression can be appropriately statistically analyzed by treatment-to-control ratios and their confidence intervals. © 2006, Drug Information Association. All rights reserved.