The invasive transformation of A-459 lung epithelial carcinoma cells has been linked to the autocrine regulation of malignant phenotypic changes by transforming growth factor β (TGF-β). Here we demonstrate, using stable ¹³C glucose isotopes, that the transformed phenotype is characterized by decreased CO₂ production via direct glucose oxidation but increased nucleic acid ribose synthesis through the nonoxidative reactions of the pentose cycle. Increased nucleic acid synthesis through the nonoxidative pentose cycle imparts the metabolic adaptation of nontransformed cells to the invasive phenotype that potentially explains the fundamental metabolic disturbance in tumor cells: highly increased nucleic acid synthesis despite hypoxia and decreased glucose oxidation.