In this study, we investigated the relationship between multiple sclerosis lesion volumes measured from magnetic resonance imaging scans and image-slice thickness. The lesion volume was computed using a semiautomated thresholding technique from axial scans of the brain of varying slice thickness. Ten patients were studied, and in all cases the computed lesion volume increased with decreasing slice thickness (p = 0.01). Linear extrapolation from our data allowed the lesion volume at very small slice thickness to be estimated; this was found to be on average 20% greater than that obtained using a slice thickness of 5 mm. Furthermore, there were considerable differences in the percentage of change in lesion volume from patient to patient, and it would appear that there is a larger variation with slice thickness for patients with smaller lesions and higher lesion loads.