Tumour-cell proliferation is a hallmark of the malignant phenotype. Positron emission tomography (PET) offers a unique method of imaging biological and biochemical changes in vivo. Radiolabelled thymidine and thymidine analogues are currently in development as PET tracers. By studying the uptake and kinetics of such compounds using PET, a measure of DNA synthesis and hence cell proliferation can be obtained. Molecular imaging of cellular proliferation with PET is now possible, and has the potential to play an important role in the evaluation of efficacy of new anti-cancer agents.