Total volume determination is important not only in experimental studies, but in clinical diagnosis, therapy and follow-up as well. Often only an approximation of the shape and size of the structure under examination is obtainable. This approximation is usually a subjective assessment of a particular structure or its components. Total volume, which is an indicator of the structural integrity of an organ or the volumetric alterations of its components, may be more accurately assessed using computed tomography (CT) scanning or magnetic resonance imaging (MRI). As a result of these assessments, conclusions may be drawn with regard to the pathologies associated with the structure of interest that may be relevant to the planning and evaluation of subsequent therapy. The training and experience of the physician is crucial during the evaluation of CT or MRI examinations. Given that the assessments made are indeed appropriate, they may nevertheless provide only limited quantitative information due to their inherently subjective nature. Recently, the Cavalieri principle (CP), which is a stereological volume estimation method, has been applied as an aid in the assessment of radiological images. It has been shown that the volume of normal and/or pathological structures capable of being delineated from neighboring structures on CT or MRI may be readily estimated by utilizing CP. In this study, the applicability of CP on CT scans or MRI is discussed and a sample calculation of vertebral body (L1) volume using lumbar vertebral images on CT is presented. Copyright © 2005 by Türkiye Klinikleri.