Objectives: Clear, objective, quantitative and early indicators of Alzheimer disease (AD) are important for clinical assessment and monitoring of therapeutic interventions. In this article, we present a recently developed post processing technique to segment and visualize the ventricular system and measure ventricular volumes in patients with mild cognitive impairment (MCI) and AD. We applied this method to quantify temporal horn volumes (THV) and the temporal horn index (THI, ratio of temporal horn volume to lateral ventricular volume) and compared the results with both a direct measurement of the hippocampal formation, and neuropsychological test performance. The aim of this article is to present the two most common methods of direct and indirect measurement and their possible limitations in comparison to the neuropsychologic scoring. Methods and materials: All subjects (21 healthy controls, 21 patients with MCI and 10 patients with AD) underwent extensive neuropsychological testing and were scanned using a 1.5 T MRI (Magnetom Vision, Siemens, Germany). A direct measurement of the hippocampal formation was produced via high-resolution T1-weighted volume imaging, while indirect temporal horn volume (THV) was calculated using a watershed algorithm-based software package (MeVis, Germany). Results: In all subjects, a successful direct and indirect quantification of the hippocampus was performed or the THV and THI were calculated. For both the manual and the semi-automated approach, a significant volume difference was found between healthy volunteers and AD patients (p < 0.001). Group analysis also showed a significant difference between the THV as well as THI of AD patients compared to MCI subjects and controls (p < 0.005). There was a significant negative correlation between the neuropsychological performance and both THV and THI (p < 0.01) as well as a significant correlation of direct and indirect measurement (p < 0.001). Conclusion: The fast and rapid development of post processing imaging techniques now allow an easy and objective method of performing hippocampal volumetry. These recently developed post processing techniques presented accurate results which are comparable to the conventional direct approach, while offering a faster and more automated technique of assessing THV and THI that could be used as an indicator of AD in a clinical context and may be helpful in studying therapeutic intervention.