The aim of this review is to summarize present evidence of an association between circulating levels of C-reactive protein (CRP) and cancer risk, and to evaluate whether elevated circulating CRP levels cause cancer. Additionally, the review provides background information on the acute-phase response, chronic inflammation, the molecular biology, function and measurement of CRP, circulating levels of CRP in health and disease, the principle of Mendelian randomization, the association between circulating levels of CRP and cancer prognosis, and cancer biomarkers. In the Copenhagen General Population Study of approximately 63,500 individuals, the distribution of circulating levels of CRP was markedly skewed to the right with 97% of the participants having CRP levels3¿mg/L) had an 80% greater risk of early death compared with those with low CRP levels (3¿mg/L at diagnosis had a 1.7-fold increased risk of death from breast cancer compared to patients with CRP levels