The microenvironment in cancerous tissues is immunosuppressive and pro-tumorigenic, whereas the microenvironment of tissues affected by chronic inflammatory disease is pro-inflammatory and anti-resolution. Despite these opposing immunological states, the metabolic states in the tissue microenvironments of cancer and inflammatory diseases are similar: both are hypoxic, show elevated levels of lactate and other metabolic by-products and have low levels of nutrients. In this Review, we describe how the bioavailability of lactate differs in the microenvironments of tumours and inflammatory diseases compared with normal tissues, thus contributing to the establishment of specific immunological states in disease. A clear understanding of the metabolic signature of tumours and inflammatory diseases will enable therapeutic intervention aimed at resetting the bioavailability of metabolites and correcting the dysregulated immunological state, triggering beneficial cytotoxic, inflammatory responses in tumours and immunosuppressive responses in chronic inflammation.