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 levels<10 mg/L. The median plasma CRP concentration was 1.53 mg/L (IQR, 1.14-2.51) and 34% of the participants had circulating CRP levels of ≥2 mg/L. Epidemiologic studies suggest that in patients with several types of solid cancers, elevated circulating levels of CRP are associated with poor prognosis, whereas in apparently healthy individuals from the general population, elevated levels of CRP are associated with increased future risk of cancer of any type, lung cancer, and possibly colorectal cancer, but not breast or prostate cancer. The robust association between circulating levels of CRP and cancer risk may be due to (1) causality: elevated CRP levels cause cancer, (2) reverse causality: occult cancer increases CRP levels, (3) or confounding: a third factor, e.g. inflammation, increases both CRP levels and the risk of cancer. Genetic epidemiologic studies (Mendelian randomization studies), which have examined the association between genetic polymorphisms influencing circulating levels of CRP and cancer risk suggest that circulating levels of CRP do not cause cancer. A lack of causality between elevated CRP levels and increased cancer risk does, however, not invalidate the potential clinical use of slightly increased CRP levels to predict risk of certain cancer types, and to improve staging and treatment allocation in patients diagnosed with cancer. Indeed, in a study of the general population, individuals with CRP levels in the highest versus the lowest quintile had a 1.3-fold increased risk of cancer of any type and a 2-fold increased risk of lung cancer. Among individuals diagnosed with cancer during the study period, individuals with a high baseline CRP (>3 mg/L) had an 80% greater risk of early death compared with those with low CRP levels (<1 mg/L). Accordingly, patients with invasive breast cancer and 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<1 mg/L at diagnosis.