PURPOSE: To evaluate the clinical usefulness of perfusion MR imaging by comparing with T2-weighted and dif-fusion weighted MR imaging in patients with acute cerebral ischemic infarction. MATERIALS AND METHODS: Conventional, diffusion weighted, and perfusion MR images were obtained within one week of clinical onset in 14 cases of acute ischemic infarction. For perfusion MRI, the gradient-echo EPI technique after IV bolus injection of 15 cc of contrast media was used. Four kinds of perfusion MR images (rCBV, rCBF, mean transit time[MTT], time to peak concentration [TTP]) were generated by home-made soft-ware from the raw data. T2-weighted, diffusion-weighted, and perfusion images of each patient were retro-spectively analyzed, with attention to the number, signal intensity, and size of lesions. RESULTS: T2-weighted and diffusion-weighted images demonstrated 21 acute ischemic lesions in 14 patients. Six lesions had a long diameter of more than 3 cm, while the other 15 were smaller than 3 cm. On T2-weighted images, 17 lesions showed high signal intensity and four showed subtle high signal intensity. On diffusion-weighted images, all lesions showed bright high signal intensity. The six lesions larger than 3 cm were all delin-eated by all four kinds of perfusion MR imaging, but among the 15 smaller than 3 cm, only four (26.7%), five (33.3%) and six (40%) were delineated on rCBV and rCBF maps, the MTT map, and the TTP map, respective-ly. As compared with T2-weighted and diffusion-weighted imaging, the rCBV and rCBF maps showed that four lesions were smaller and six were the same size. On the MTT map, three lesions were seen to be larger, four were smaller, and the other four were the same size as they appeared on diffusion-weighted images, while on the In TTP map, seven were larger and five were smaller than they appeared on these images. CONCLUSION: In all cases, diffusion-weighted images most clearly delineated acute ischemic lesions, regardless of lesion size. Many such lesions smaller than 3 cm were not apparent on perfusion MR images. Among the four kinds of perfusion MR imaging, TTP and MTT maps may be clinically useful for evaluation of the penum-bral zone in cases of acute cerebral ischemic infarct.