Addressing the HIV/AIDS epidemic requires an understanding of how risky sexual behaviours change over time. This column observes that, whereas the accuracy of self-reported data depends on the likelihood of people telling the truth, the likelihood of risky behaviours being detected in tests for sexually transmitted infections is equal to the disease transmission rate. Self-reported data may therefore be a more reliable measure of risky behaviours than the prevalence of sexually transmitted infections when the probability of transmission is low.