We examined whether there was any difference in the value, and temporal evolution, of the apparent diffusion tensor trace ((ADC) over bar) in acute and subacute grey and white matter lesions. Thirty-seven patients underwent diffusion imaging once (up to 3 days), 15 patients were scanned Mice (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) over bar (<(ADCr)over bar>) 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)over bar> 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)over bar> for grey matter rose significantly (p = 0.02) from <(ADCr)over bar> similar to 0.7 (< 7 days) to 0.95 (greater than or equal to 10 days). This increase did not occur in white matter whose <(ADCr)over bar> remained fairly constant (<(ADCr)over bar> similar to 0.7) over the time course of the study. NeuroReport 11:2867-2874 (C) 2000 Lippincott Williams & Wilkins.