The mean apparent diffusion coefficient (ADC) of the human optic nerve (ON) has been quantified in vivo, and mean ADC maps are shown along the complete length of the nerve from the globe to the optic chiasm. The mean ADC, over the whole nerve, is shown to be 1058 x 10(-6) mm(2) s(-1) (standard deviation (SID), over nine 3-mm slices, 101 x 10(-6) mm(2) s(-1); range (833-1178) x 10(-6) mm(2) s(-1)). The robustness of the method relies on acquisition of high-resolution coronal images of the ON using the ZOOM-EPI technique, which makes use of a shortened echo train length for increased resolution with decreased susceptibility-induced distortions. Suppression of the cerebrospinal fluid (CSF) and fat signals from tissues that surround the ON also helps successful identification and delineation of the nerve. Averaging of magnitude images is used to compensate for the inherently low signal-to-noise ratio (SNR) of the acquired images; the effects of the Rayleigh distributed noise in such images are allowed for during ADC calculations. Magn Reson Med 47:24-31, 2002. (C) 2002 Wiley-Liss, Inc.