Uncontrolled growth is a characteristic of malignant tumors. Histochemical techniques to measure tumor growth rate in tissue specimens have proved useful, but are limited because of sampling and the difficulty of following response to therapy. PET imaging offers the opportunity to measure tumor growth non-invasively and repeatedly as an early assessment of response to therapy. Measuring cellular growth instead of energy metabolism offers significant advantages in evaluating therapy. The rationale is that a cell's biosynthetic machinery, rather than its fueling process, is more susceptible to cancer therapy. Cytostatic agents may not reduce the quantity of viable tumor; so imaging a change in cellular proliferation may be the only effective way to assess the response to therapy. Radiopharmaceuticals to image growth include labeled amino acids, lipid precursors, and nucleosides. The biochemical characteristic that most uniquely distinguishes successfully treated cancer cells is that they no longer synthesize DNA and no longer divide. Thus imaging with labeled thymidine, which is incorporated into DNA but not into RNA, provides definitive evidence of a cell that is proliferating and, therefore, whether it has responded to treatment.