['To assess the apparent diffusion coefficient (ADC) derived from diffusion-weighted (DW) magnetic resonance imaging (MRI) as a specific marker of renal fibrosis in rats with unilateral ureteral obstruction (UUO).', 'Thirteen rats were analyzed in group 1 (n\u2009=\u20094), group 2 (n\u2009=\u20093), and group 3 (n\u2009=\u20096) and measured using a clinical 3.0T MR scanner. Groups 1 and 2 were used to establish the final imaging protocols for group 3. DW imaging with four b-values (0, 50, 300, 800 s/mm(2) ) was conducted before UUO, at days 3 and 5 after UUO, after release of the obstruction, and after sacrifice. Renal cortical ADCs were correlated with histological and ultrastructural analyses.', 'ADC values of group 3 are shown as mean\u2009±\u2009standard deviation of [10(-3) mm(2) /s]. On day 5, in vivo cortical ADC of obstructed fibrotic kidneys was significantly reduced compared to unobstructed kidneys (1.4\u2009±\u20090.086 vs. 1.535\u2009±\u20090.087, P\u2009=\u20090.0018). Postmortem ADC dropped by 50% and was significantly increased in obstructed vs. unobstructed kidneys (0.711\u2009±\u20090.094 vs. 0.566\u2009±\u20090.049, P\u2009=\u20090.0046). Histopathology of obstructed kidneys showed tubular dilation, tubular cell atrophy, and expansion of the interstitial space. Postmortem ADC correlated tightly with tubular lumen area (r\u2009=\u20090.9, P\u2009<\u20090.001), fibronectin (r\u2009=\u20090.8, P\u2009=\u20090.003), collagen type I (r\u2009=\u20090.73, P\u2009=\u20090.007), and interstitial expansion (r\u2009=\u20090.69, P\u2009=\u20090.013).'] Does diffusion-weighted MRI reflect kidney fibrosis in a rat model of fibrosis?