Primary care data in the UK are widely used for cancer research, but the reliability of recording key events like diagnoses remains uncertain. Although data linkage can improve reliability, its costs, time requirements, and sample size constraints may discourage its use. This report evaluated accuracy, completeness, and date concordance of prostate cancer (PCa) diagnosis recording in Clinical Practice Research Datalink (CPRD) GOLD and Aurum compared to linked Cancer Registry (CR) and Hospital Episode Statistics (HES) Admitted Patient Care (APC) in England.