Although tetanus is now rare, vaccination is currently recommended for the entire population. Most elderly North Americans have never received tetanus vaccination. We evaluated the expected cost-effectiveness of using mailed reminders from family physicians to increase primary tetanus vaccination coverage among elderly Canadians. We estimated that over 10 years the program would prevent five cases of tetanus and one death from tetanus, resulting in a gain of 13 life-years. There would be 16,700 adverse reactions to tetanus toxoid, 17% in people already immune to tetanus. The net cost of the program (in 1984 Canadian dollars) would be $1.9 million per case of tetanus prevented, $7.1 million per death prevented and $810,000 per life-year gained. These high cost-effectiveness ratios are largely attributable to the very low risk of tetanus, even among nonimmune elderly people. Tetanus toxoid and physicians' services for vaccination would account for 86% of the program costs. Because the mailed reminders would be responsible for only 13% of the program costs, other possible programs to increase primary tetanus vaccination coverage could not be expected to have substantially lower cost-effectiveness ratios. We conclude that efforts to increase primary tetanus vaccination coverage among elderly Canadians would be a questionable use of health care resources.