To evaluate diffusion-weighted magnetic resonance imaging (DW-MRI) as a marker for disease aggressiveness by comparing tumour apparent diffusion coefficients (ADCs) between patients with low- versus higher-risk localized prostate cancer. Forty-four consecutive patients classified as low- [n = 26, stageT1/T2a, Gleason score < or = 6, prostate-specific antigen (PSA)< 10 (group 1)] or intermediate/high- [n = 18, stage > or = T2b and/or Gleason score > or = 7, and/or PSA > 10 (group 2)] risk, who subsequently were monitored with active surveillance or started neoadjuvant hormone and radiotherapy, respectively, underwent endorectal MRI. T2-weighted (T2W) and DW images (5 b values, 0-800 s/mm(2)) were acquired and isotropic ADC maps generated. Regions of interest (ROIs) on T2W axial images [around whole prostate, central gland (CG), and tumour] were transferred to ADC maps. Tumour, CG, and peripheral zone (PZ = whole prostate minus CG and tumour) ADCs (fast component from b = 0-100 s/mm(2), slow component from b = 100-800 s/mm(2)) were compared. T2W-defined tumour volume medians, and quartiles were 1.2 cm(3), 0.7 and 3.3 cm(3) (group 1); and 6 cm(3), 1.3 and 16.5 cm(3) (group 2). There were significant differences in both ADC(fast) (1778 +/- 264 x 10(-6) versus 1583 +/- 283 x 10(-6) mm(2)/s, p = 0.03) and ADC(slow) (1379 +/- 321 x 10(-6) versus 1196 +/- 158 x 10(-6) mm(2)/s, p = 0.001) between groups. Tumour volume (p = 0.002) and ADC(slow) (p = 0.005) were significant differentiators of risk group. Significant differences in tumour ADCs exist between patients with low-risk, and those with higher-risk localized prostate cancer. DW-MRI merits further study with respect to clinical outcomes.