We examined whether there was any difference in the value, and temporal evolution, of the apparent diffusion tensor trace (ADC) in acute and subacute grey and white matter lesions. Thirty-seven patients underwent diffusion imaging once (up to 3 days), 15 patients were scanned twice (up to 7 days), and seven patients were scanned three times (up to 14 days) after stroke. Values of the ratio of ischaemic to contralateral ADC (ADCr) were reduced on average by 30% (p<0.001) in the whole hyperintense region up to 7 days post-ictus. No difference was seen between ADCr values of grey and white matter in individual subjects within the patient groups scanned up to 7 days. However, in the subgroup of patients scanned beyond 7 days, ADCr for grey matter rose significantly (p=0.02) from ADCr approximately 0.7 (< 7 days) to 0.95 (> or = 10 days). This increase did not occur in white matter whose ADCr remained fairly constant (ADCr approximately 0.7) over the time course of the study.