The present status of nickel (7440020) carcinogenesis is reviewed, with particular attention to the route of entry of nickel compounds into target cells and the possible mechanisms of nickel initiated neoplastic transformation. To date, more than 327 cases of lung cancer and 115 cases of nasal cancer have occurred among workers occupationally exposed to nickel in seven countries. Several nickel compounds are probably carcinogenic for man following chronic exposures by inhalation. Mortality studies have demonstrated enhanced prevalence of cancers of the respiratory tract among nickel workers. Certain groups of nickel workers have been subjected to special risks of developing respiratory cancers. Cancers of the respiratory tract may represent an occupational hazard among workmen involved in nickel plating and grinding operations. Among Welsh nickel workers, the average latent period of lung cancer is 4 years longer than that for nasal cancer. There is a shortage of epidemiological data on the carcinogenic interaction of cigarette smoking and occupational nickel exposure, and there is little evidence of nonrespiratory cancers in nickel workers. Experimental models of nickel carcinogenesis are summarized. Fifteen nickel compounds have been tested for carcinogenicity in rats following parenteral injection, and the valences, solubilities and relative carcinogenicities have been compared. Investigations of the carcinogenicity of nickel-carbonyl (13463393) are summarized. Several studies on the induction of sarcomas in rats by intramuscular or subcutaneous injections of metallic nickel in the forms of pellets, dust, or sponges are summarized. Intracellular transport of nickel compounds, mechanisms of nickel carcinogenesis, nickel as a possible carcinogen in tobacco, and carcinogenic hazards of nickel implants are discussed. The author concludes that there is clear evidence of carcinogenicity for certain nickel compounds.