Effective interventions for care of health need to be based on scientific evidence. To this end, the Cochrane Collaboration insists that its reviews should be based on reliable data, normally obtained by randomised controlled trial. To constitute evidence, data should also support a hypothesis in accord with scientific laws and knowledge. From these considerations, an appraisal is made of the conclusion of the Cochrane review Helmets for preventing head and facial injuries in bicyclists, that it establishes scientific evidence that all types of standard helmet protect against injuries to the brain. It is concluded that the review takes no account of scientific knowledge of types and mechanisms of brain injury. It provides, at best, evidence that hard-shell helmets, now rarely used, protect the brain from injury consequent upon damage to the skull. The review therefore is not a reliable guide to the efficacy of helmets and to interventions concerning their use.