Following the publication of the NIOSH nickel criteria document in 1977, the Joint Occupational Health Committee of the International Nickel Company (INCO) commissioned a mortality study of the company's Ontario workforce. This paper describes the detailed methodology and primary mortality results of the ensuing study; subsequent papers will describe more detailed findings of cause-specific mortality. An historical prospective mortality study of approximately 54,000 INCO workers has been conducted. Men with six months or more of service were followed for mortality during a 35-year period by computerized record linkage to the Canadian National Mortality Data Base. From a company-provided list of men known to have died and through independent follow-up of a random sample of 1,000 subjects of unknown status, we estimate a mortality ascertainment rate of 95%. Cause-specific standardized mortality ratios calculated with respect to Ontario provincial mortality rates indicate an excess of accidental deaths in men working in the Sudbury area and an excess of cancer deaths at the company's Port Colborne nickel refinery. A strong healthy worker effect was found for both all-disease mortality ad cancer mortality. The lower than expected mortality persisted for about 15 years beyond initial hiring.