The purpose of this study was to clarify the effects of cohort formation, period of entry, and period of follow-up on occupational mortality figures. The study material comprised three cohorts of metal industry workers (6 415 iron foundry workers, 3 901 metal product workers, and 5 398 workers manufacturing electrical devices). The men, hired between 1950 and 1976, were followed until the end of 1978. The number of person-years was approximately 215 800. Different cohort formation criteria and variations in the follow-up modify the results of occupational mortality studies. A cross-sectionally based cohort (workers employed in a certain year or years) produces results different from those for an open cohort (new workers hired during a certain period of time) although the cohorts are formed from the same workplaces. The cohorts for retrospective cohort studies usually contain various periods of entry, periods of follow-up, and age structures. Increased or decreased mortality in a cohort depends on this internal structure. The structure of the cohort becomes especially important when a certain cause of death is concentrated in certain age classes and/or when a period of latency is required, as for tumors. In an attempt to reveal this internal structure the three cohorts were analyzed in five-year calendar periods and stratified according to the length of follow-up. The mortality pattern remained nearly unchanged when the number of periods of entry increased, and it was also comparable with the mortality pattern of the general male population.(ABSTRACT TRUNCATED AT 250 WORDS)