We assessed the reliability of an interactive 3-dimensional methodology for magnetic resonance imaging (MRI) brain tissue segmentation and volumetry using a 3-dimensional magnetization prepared rapid gradient echo pulse sequence (3D MP-RAGE). The methodology was intended to be practically useful to study structural brain changes in larger groups of patients investigated for suspected dementia. The pulse sequence combines volume acquisition, excellent tissue contrast and short patient scan-time. The volumetric method is fully interactive, requiring a minimum of image pre-processing. Ten healthy controls and 10 patients with dementia were included in the study. Six healthy controls were scanned twice. The method is based on thresholding combined with manual tracing in a 3D volume. The 3-dimensional measurement method reduces the measurement time considerably compared to that of slice by slice measurement and permits 3-dimensional display of measured volumes. For different brain regions the intra-study (0.5-1.3%), study-study (1.8-4.7%) and inter-operator (7.1%) variability of this method compared favourably with other manual or automated methods reported. The major advantages of the method are its simplicity and speed, which permits measurement and display of regional brain volumes and tissues in larger patient groups.