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The number of patients who need to be treated to prevent one additional event (number needed to treat; NNT) has become a widely used measure of treatment benefit derived from the results of randomised controlled trials with a binary outcome. 1 2 We show how to obtain a number needed to treat for studies where the primary outcome is the time to an event. We consider primarily the situation where there is no access to raw data, for example, when reviewing a published study, and also how to proceed when given the raw data.