Objective: To evaluate the effectiveness and cost of obtaining consent for review of medical records within the passively observed non-intervention arm of a cluster randomized controlled trial, ‘Comparison Arm for ProtecT’. Methods: Two hundred and thirty men, who had been notified to the trial by cancer registries as having prostate cancer, were sent a consent form from their general practitioner or secondary care clinician. The consent rate of participants to the review of their medical records and the estimated costs of the process were evaluated. Results: One hundred and seventy-nine men (84%: 95% CI = 78%, 89%) consented to have their medical notes reviewed at an estimated cost of £123 (€172, $248) per person. Conclusions: A high consent rate for review of medical notes is achievable but at a cost. There needs to be renewed debate about the automatic need for consent to review medical records where the chance of personal harm is negligible and the purpose of the review is to provide robust evidence to save lives, prevent needless suffering, and improve the effectiveness and efficiency of health care delivery.