The follow-up of a cohort of workers employed in an electrochemical plant producing cobalt and sodium, previously studied from 1950-1980, has been extended from 1981-1988. The standardized mortality ratio (SMR) for all causes of death was 0.85 (95% confidence interval [CI] = 0.76-0.95, 309 observed) for the whole cohort, and 0.95 (95% CI = 0.83-1.08, 247 observed) for the subcohort of workers born in France. With regard to lung cancer mortality among cobalt production workers, which is the main objective of the study, the SMRs were, respectively, 0.85 (95% CI = 0.18-2.50, 3 observed) and 1.16 (95% CI = 0.24-3.40, 3 observed). Neither did any excess of mortality from diseases of the circulatory and of the respiratory systems appear among cobalt production workers. Maintenance workers, however, exhibited high SMRs for lung cancer, reaching statistical significance for duration of exposure and time since first exposure > or 30 years. This study does not support the hypothesis of a relationship between lung cancer and cobalt exposure.