Colloid cysts of the third ventricle are rare benign cysts but they may be potentially life-threatening. Three cases of sudden death resulting from colloid cysts of the third ventricle are presented. The first and second cases were treated for migraine headaches. In the first case, the patient was a 24-year-old woman who presented to the hospital with a severe headache and was sent back home after medical treatment. Six hours later, she was found dead in her bed. The second case was a 21-year-old woman who experienced a severe headache, dizziness and vomiting 1 day prior to her death. She was transported to the hospital, where she was pronounced dead upon arrival. The third case was a 25-year-old man who experienced headaches and vomiting and was diagnosed with and medically treated for sinusitis. He lost consciousness and was taken to hospital, where he was pronounced dead on arrival. During the autopsy of all three cases, there was a grey transillumination area observed that occurred due to the stretching of tissue at the base of brain between the corpus mamillare and chiasma opticum. Dissection of the brain revealed a colloid cyst of the third ventricle. To avoid such fatal complications, prompt diagnosis using CT or MRI is essential in patients who have a long-standing history of intermittent headaches. During the autopsy of the sudden deaths of people with medical antecedents of headaches, if a grey color is observed between the chiasma opticum and the corpus mamillare in the base of the brain, a colloid cyst should be considered and this region should be dissected and examined carefully. © 2009 Elsevier Ireland Ltd. All rights reserved.