PURPOSE: To compare the detection rate of unenhanced CT, spin-echo T2-weighted, fast fluid-attenuatedin-version- recovery(FLAIR), and diffusion-weighted MR imaging in the diagnosis of hyperacute middle cerebralartery(MCA) territory infarction. MATERIALS AND METHODS: Sixteen patients with clinically proven hyperacute MCAterritory infarction were e-valuated with unenhanced CT and MR. All CT examinations were performed within sixhours of the onset of symptoms and all MR studies were performed within two hours of CT. All images were evaluatedindepen-dently by two radiologists in possession of brief clinical information. Positive imaging criteria wereparenchy-mal hypoattenuation, as seen on CT, and increased signal intensity, as seen on MR. For quantitativeanalysis, we measured the attenuation and signal intensity of the lesion and contralateral normal parenchyma, andper-centage contrast-to-noise ratios(CNRs) of the lesions were also calculated. RESULTS: Positive findings weredetected in all patients on diffusion-weighted images, in 13(81%) on CT, in 10 ( 63 %) on fast FLAIR images, andin 7(44 %) on T2-weighted images. Lesion percentage CNRs were 30% for diffusion-weighted imaging, 15 % for CT, 18% for FLAIR MR imaging, and 16 % for T2-weighted MR imag-ing(p < .004 for diffusion-weighted imaging vs others). CONCLUSION: For hyperacute MCA territory infarction, diffusion-weighted MR imaging was the most sensitive imagingtechnique and unenhanced CT was superior to fast FLAIR or T2-weighted imaging.