The efficacy of intravenous mecobalamin in the treatment of Alzheimer-type dementia was evaluated in ten patients using several rating scales. Vitamin B12 levels and unsaturated binding capacities were also measured and compared to the evaluated intellectual function scores. Mecobalamin was shown to improve intellectual functions, such as memory, emotional functions, and communication with other people. Improvements in cognitive functions were relatively constant when the vitamin B12 levels in the cerebrospinal fluid were high. Improvements in communication functions were seen when a certain level of vitamin B12 was maintained for a longer period. There were no side effects attributable to mecobalamin. We conclude that mecobalamin is a safe and effective treatment for psychiatric disorders in patients with Alzheimer-type dementia.