This study systematically analyzed tumour purity across 21 cancer types using four methods and a consensus approach. Tumour purity refers to the proportion of cancer cells in the tumour microenvironment, which includes immune cells, fibroblasts, and endothelial cells. The study used data from the Cancer Genome Atlas (TCGA) to assess tumour purity in over 10,000 samples. The results showed that tumour purity varies significantly between cancer types and is influenced by both intrinsic and extrinsic factors. Intrinsic factors are characteristics of the tumour itself, while extrinsic factors relate to sample collection methods. The study found that tumour purity can confound genomic analyses, particularly in co-expression networks and differential expression analyses. Adjusting for tumour purity revealed new insights into immune-related pathways, including those involved in immunotherapy. The study highlights the importance of accounting for tumour purity in cancer research to avoid misinterpretation of genomic data. The findings suggest that tumour purity is an important factor in cancer biology and that future studies should consider it when analyzing genomic data. The study also emphasizes the need for further research to better understand the effects of tumour purity on molecular subtyping and clinical outcomes.This study systematically analyzed tumour purity across 21 cancer types using four methods and a consensus approach. Tumour purity refers to the proportion of cancer cells in the tumour microenvironment, which includes immune cells, fibroblasts, and endothelial cells. The study used data from the Cancer Genome Atlas (TCGA) to assess tumour purity in over 10,000 samples. The results showed that tumour purity varies significantly between cancer types and is influenced by both intrinsic and extrinsic factors. Intrinsic factors are characteristics of the tumour itself, while extrinsic factors relate to sample collection methods. The study found that tumour purity can confound genomic analyses, particularly in co-expression networks and differential expression analyses. Adjusting for tumour purity revealed new insights into immune-related pathways, including those involved in immunotherapy. The study highlights the importance of accounting for tumour purity in cancer research to avoid misinterpretation of genomic data. The findings suggest that tumour purity is an important factor in cancer biology and that future studies should consider it when analyzing genomic data. The study also emphasizes the need for further research to better understand the effects of tumour purity on molecular subtyping and clinical outcomes.