Sudden unexpected death caused by a colloid cyst of the third ventricle in young apparently healthy persons is an uncommon phenomenon that nevertheless occurs regularly in medical examiner's cases and may occur in general hospital pathology practice. This entity has not received proper attention in the general or forensic pathology literature in spite of the fact that sudden death has been appreciated in the clinical literature for many years. We report four cases of sudden unexpected death in young persons whose only major previous complaint was that the chronic relapsing headaches. Because of the sudden and unexpected nature of their deaths, these cases came to the attention of the medical examiner and the colloid cysts were discovered at autopsy. The history of the lesion, its histogenesis, its pathological physiology, and the experiences of others are reviewed.