The Canadian Mortality Data Base (MDB) was utilized in mortality follow-up of 17,446 refinery and petrochemical workers throughout Canada. The performance of the system in detecting 757 previously known deaths was evaluated. Of the deaths submitted, 93.1% (90% confidence interval (CI) 91.5-94.5) were detected, including 97.6% (90% CI 96.5-98.4) of deaths which occurred in Canada. Detection was generally unaffected by age, year of death (1964-1973), and the presence of middle initial or the remainder of first name. Slightly lower ascertainment was found for deaths occurring in Quebec (94.5%) and Newfoundland (93.3%). This could be due to several factors, including fewer MDB records with complete identifier information for these provinces, or lower accuracy of linking fields for records supplied to the Agency. Few links were made to employees presumed to be alive, indicating 99.8% specificity for these records.