The RAS/MAPK (mitogen-activated protein kinase) signalling pathway is frequently deregulated in non-small-cell lung cancer, often through KRAS activating mutations^,,. A single endogenous mutant Kras allele is sufficient to promote lung tumour formation in mice but malignant progression requires additional genetic alterations^,,,. We recently showed that advanced lung tumours from Kras^G12D/+;p53-null mice frequently exhibit Kras^G12D allelic enrichment (Kras^G12D/Kras^wild-type > 1) (ref. ), implying that mutant Kras copy gains are positively selected during progression. Here we show, through a comprehensive analysis of mutant Kras homozygous and heterozygous mouse embryonic fibroblasts and lung cancer cells, that these genotypes are phenotypically distinct. In particular, Kras^G12D/G12D cells exhibit a glycolytic switch coupled to increased channelling of glucose-derived metabolites into the tricarboxylic acid cycle and glutathione biosynthesis, resulting in enhanced glutathione-mediated detoxification. This metabolic rewiring is recapitulated in mutant KRAS homozygous non-small-cell lung cancer cells and in vivo, in spontaneous advanced murine lung tumours (which display a high frequency of Kras^G12D copy gain), but not in the corresponding early tumours (Kras^G12D heterozygous). Finally, we demonstrate that mutant Kras copy gain creates unique metabolic dependences that can be exploited to selectively target these aggressive mutant Kras tumours. Our data demonstrate that mutant Kras lung tumours are not a single disease but rather a heterogeneous group comprising two classes of tumours with distinct metabolic profiles, prognosis and therapeutic susceptibility, which can be discriminated on the basis of their relative mutant allelic content. We also provide the first, to our knowledge, in vivo evidence of metabolic rewiring during lung cancer malignant progression.