The aim of this study was to investigate whether frontal lobe damage affects thalamic volume in humans. Ipsilateral and contralateral thalamic areas were measured in 0.5T T1-weighted sagittal magnetic resonance images in 12 patients, first at the time of their surgery for relief of a unilateral frontal lobe brain tumor and at follow-up ~2 years later. A 5% decrease in ipsilateral and 4.5% increase in contralateral thalamic area was found over time (F(I,II) = 6.15, p<0.05). We conclude that unilateral frontal lobe damage results in a decrease in the ipsilateral thalamus and an increase in the contralateral thalamus in humans in vivo. The findings may have implications for the interpretation of the reported changes in thalamic volume in neuropsychiatric diseases. (C) 2000 Lippincott Williams and Wilkins.