OBJECTIVE. The objective of this study was to determine the time course of signal intensity changes on diffusion-weighted MR images after cerebral infarction. MATERIALS AND METHODS. Echoplanar diffusion-weighted MR images were obtained at 1.5 T in 212 patients referred for suspected cerebral infarction over a 6-month period. Of those patients, 85 met strict criteria for inclusion in this study: final clinical diagnosis of stroke, reliable timing of clinical ictus by history, and neurologic symptoms persisting longer than 48 hr after onset. Using adjacent or contralateral normal brain for comparison, diffusion-weighted images were visually analyzed retrospectively to evaluate for abnormalities in signal intensity. Because three patients were scanned on two occasions and five patients had two anatomically separable infarctions, 93 reliably dated brain lesions were analyzed. RESULTS. Diffusion-weighted images showed abnormal findings in 13 (100%) of 13 lesions less than 1 day old, 46 (96%) of 48 lesions 1-4 days old, 16 (94%) of 17 lesions 5-9 days old, three (60%) of five lesions 10-14 days old, and zero (0%) of 10 lesions more than 14 days old. CONCLUSION. Abnormal signal intensity was present on all diffusion-weighted MR studies obtained in patients within 24 hr of acute cerebral infarction and in up to 94% of patients scanned during the first 2 weeks after ictus. The percentage of abnormal diffusion studies declined with time, and no signal intensity abnormality was seen in stroke patients scanned more than 2 weeks after symptom onset.