In this study, we performed a post-contrast magnetization-prepared rapid acquisition gradient echo (MP RAGE) sequence with subsequent reconstruction of axial slices with 5-, 3-, and 1-mm thickness in 26 patients with multiple sclerosis (MS) to investigate the disease activity going on below the resolution of conventional scans. One hundred three enhancing lesions were detected using MP RAGE reconstructed with 1-mm-thick slices, 93 using MP RAGE reconstructed with 3-mm-thick slices and 82 using MP RAGE reconstructed with 5-mm-thick slices. The detection rate using 1-mm MP RAGE was 25.6% higher compared with 5-mm MP RAGE and 12% higher compared with 3-mm MP RAGE. Using 1-mm MP RAGE, we detected five enhancing lesions in 2 of the 12 patients (16.6%) who showed no enhancing lesions on the 5-mm MP RAGE and one enhancing lesion in 1 of the 10 patients (10%) who showed no enhancing lesions on the 3-mm MP RAGE. The increased sensitivity of 1-mm MP RAGE over the other two reconstruction slice thicknesses was due to improved detection of small lesions, located mainly in cortical/subcortical areas. These data indicate that the size of enhancing lesions in MS is highly variable, and that a significant proportion of disease activity goes undetected when performing conventional scans.