The possible relationship between deep white matter hyperintensity (WMHI) lesions detected by magnetic resonance imaging and response to lithium was examined in 16 patients with bipolar disorder who had been under maintenance treatment with lithium for more than 1 year. Bipolar patients who had higher scores of WMHI responded significantly better to lithium (r = 0.57, P < 0.05) than did those who had lower scores. This preliminary result suggests that the presence of WMHI may be associated with a better response to lithium.