OBJECTIVE: To assess the effect of three computerized reminder systems on compliance with tetanus vaccination. DESIGN: Prospective randomized controlled trial. SETTING: Ottawa Civic Hospital Family Medicine Centre. PARTICIPANTS: Of 8069 patients 20 years of age or more who were not in a hospital or institution 5589 were randomly assigned, by family, to a control group, a physician reminder group, a telephone reminder group or a letter reminder group. The remaining 2480 patients were not included in the randomized portion of the study but were monitored. Results are presented for the 5242 randomized patients and the 2369 nonrandomized patients for whom there was no up-to-date record of tetanus vaccination at the start of the trial. INTERVENTIONS: For the patients in the physician reminder group the physician was reminded at an office visit to assess the patient's tetanus vaccination status and to recommend vaccination; those in the other two reminder groups received a telephone call or letter enquiring about their tetanus vaccination status and recommending a booster dose. MAIN OUTCOME MEASURE: Proportion of patients who received tetanus toxoid during the study year or who had a claim of vaccination in the previous 10 years. MAIN RESULTS: The rate of recorded tetanus vaccination in the randomized control group was 3.2%. The difference between that rate and those for the three reminder groups was 19.6% in the physician reminder group (95% confidence interval [CI] 17.1% to 22.2%, p less than 0.00001), 20.8% in the telephone reminder group (95% CI 18.3% to 23.5%, p less than 0.00001) and 27.4% in the letter reminder group (95% CI 24.8% to 30.2%, p less than 0.00001)). The letter reminders were more effective than either the telephone reminders (p = 0.00013) or the physician reminders (p less than 0.00001) in improving compliance. The cost to the practice per additional vaccination recorded was 43 for the physician reminders, $5.43 for the telephone reminders and $6.05 for the letter reminders. CONCLUSIONS: Although all three reminder systems increased the rate of recorded tetanus vaccination they fell far short of achieving complete population coverage. More intensive interventions would be required to approach that goal. However, such interventions do not appear to be justified given the rarity of tetanus.