OBJECTIVE: To evaluate the accuracy of patients' recall of their last Tetanus/Diphtheria (Td) booster in the setting of employee health clinics known for high immunization rates and good documentation of vaccine status. METHODS: Five hundred and seventy-two patients of an employee health clinic answered a written questionnaire about whether they have had a Td booster in the last 10 years. Answers were compared with patients' charts as the gold standard. RESULTS: The sensitivity of the question is 92.4% (95% CI 89.0-95.0) and specificity is 26.5% (95% CI 12.9-44.4). Positive predictive value is 92.6% (95% CI 89.3-95.2) and negative predictive value is 25.7% (95% CI 12.5-43.3). Age and gender do not affect the accuracy of recall. CONCLUSION: A positive answer to this question is highly reliable whereas a negative answer is unreliable. PRACTICE IMPLICATIONS: In the setting of employee health clinics, when patients affirm the receipt of a Td booster within the previous 10 years, it is quite likely that they are up to date and do not require re-administration of the vaccine. Any other answer to this question (No, I do not know) is unreliable and a booster should be recommended if not contraindicated.