A continuous follow-up review of colloid cysts including aspects of natural history and evaluation of treatment options is necessary to optimize individual treatment. Thirty-seven consecutive patients with colloid cyst of the third ventricle seen at Karolinska Hospital between 1984 and 1995 were reviewed. Five patients were admitted in a comatose state, and two died despite emergency ventriculostomy. Three had recurrent cysts following previous aspiration procedure. During the study period, patients underwent a total of 10 ventriculostomies, 10 aspirations, 26 microsurgical operations, and two shunt operations. Twenty-four of 26 microsurgical operations were transcallosal and two were transcortical. Twenty-four operations (22 transcallosal and two transfrontal approaches) without permanent morbidity were performed by four surgeons. Transient memory deficit from forniceal traction was noted in 26%. The remaining two transcallosal operations, which led to permanent morbidity or mortality, were performed by two different surgeons. Aspiration of cysts performed by four different surgeons carried a 40% risk of transient memory deficit (10% permanent) and an 80% recurrence rate. One patient was found to be cured on radiological studies obtained at the 5-year follow-up review. Seven cysts were followed by means of radiological studies with no treatment for 6 to 37 months. Five of these cysts grew, indicating that younger patients with colloid cysts will probably need surgical treatment. The main causes of unfavorable results were: 1) failure to investigate symptoms that proved fatal; 2) subtotal resection; and 3) surgical complications. Transcallosal microsurgery produced excellent results when performed by experienced surgeons. A colloid cyst of the foramen of Monro is a disease that should be detected before permanent neurological damage has occurred. Permanent morbidity or mortality should not be accepted in modern series of third ventricle colloid cysts.