Axonal loss is likely to be an important component of atrophy and the pathological substrate for the fixed disability of MS. To estimate the rate of central white matter reduction we investigated ventricular volume change and disease activity on monthly MRI in 19 patients over 6 months. At baseline, ventricular volumes were largest in primary progressive MS and smallest in relapsing–remitting MS. Over the study period ventricular volumes increased overall by 0.2% (F=2.75, P=0.02), but the percentage changes in relapsing–remitting MS were much larger (median increase 14.9%). Lesion volumes were also highest at baseline in relapsing–remitting MS, but serial changes in ventricular volumes were not correlated with serial changes in lesions. This study shows that ventricular enlargement in MS may occur over short epochs, particularly in relapsing–remitting cases. However, the loss of central white matter volume observed in any brief period may be related to inflammatory activity that occurred in a preceding or earlier epoch, a delayed post‐inflammatory degenerative process, or most likely, a combination of both.