A self-administered algorithm for the assessment of symptoms of, and risk factors for, sexually transmitted infection (STI) was developed. The algorithm was applied retrospectively to all cases of STI other than chlamydia diagnosed over a one-year period, to estimate the risk of missing STI diagnoses by its introduction. Three hundred and eighty-nine cases were identified, of whom 81 (21%) were asymptomatic. Seven asymptomatic cases had no identifiable risk factors for infection and might potentially have been offered a postal testing kit for chlamydia rather than a full STI screen. We estimate that 1.8% of STIs diagnosed in the unit might be missed by the introduction of the algorithm.