Cortical lesions (CLs) can be detected in the majority of patients with established multiple sclerosis (MS), but little is known about their evolution over time. This study was performed to investigate the short-term MRI evolution of CLs, with the ultimate aim to achieve a better in vivo understanding of their nature. Seven hundred and sixty-eight CLs from 107 MS patients (76 with relapsing-remitting [RR] and 31 with secondary progressive [SP] MS) were followed with brain MR examinations, including a double inversion recovery (DIR) sequence, every 6 months for 1 year. CLs' number, volume and morphological features were assessed at each time-point. Six hundred and eighty CLs (88.5%) remained morphologically unchanged during the follow-up period, while 74 (9.6%) showed an increase in size. Only 6 (0.8%) CLs seen at baseline (all in RRMS patients) disappeared at follow-up MRI scans. No enlarging CLs spread into the subcortical white matter. No CLs ever showed gadolinium enhancement. At baseline, the mean number of CLs was higher in SPMS than in RRMS patients (p<0.001), whereas the mean number of new CLs per patient after 1 year did not differ between the two groups. Over a one-year period, CLs can increase their number and size in a relevant proportion of MS patients, without spreading into the subcortical white matter or showing inflammatory features similar to those of white matter lesions. The short-term rate of CLs accumulation does not seem to vary according to the clinical stage of MS.