Cobalt and nickel are classified as micro-trace elements in the human body. Cobalt is essential in nutrition as a component of vitamin B12 whilst nickel has a probable role in RNA transcription, lipid and carbohydrate metabolism. These elements exhibit ferromagnetic properties but are biologically dissimilar. They are weakly mutagenic in bacterial tests, but clinical and experimental evidence links inhalation of cobalt or nickel dust to an increased risk of lung cancer in exposed workers, possibly through epigenetic mechanisms involving oxygen stress and production of reactive oxygen species. Animal studies are inconsistent. Co(II) and Ni(II) are potentially carcinogenic following inhalation and parenteral injection in rats. Carcinogenic risk has not been identified clinically in patients implanted with orthopaedic prostheses containing cobalt–nickel alloys. Cigarette smoke contains cobalt and nickel residues and may contribute to pulmonary carcinogenesis.