The objective of this study is to estimate the value of diffusion-weighted MRI (DWI) in the detection of abnormalities in sacroiliac joints in the patients with early ankylosing spondylitis (AS) and investigate the feasibility of whole-body DWI (WB-DWI) in systemic evaluation of AS. Sixteen patients with early AS, 18 patients with simple low back pain (LBP), and 18 healthy volunteers were involved in this study. All subjects underwent conventional MRI and DWI. Apparent diffusion coefficient (ADC) in subchondral bone marrows of sacroiliac joints was measured. Independent-sample t test was used to statistically analyze the difference of ADC values between groups. WB-DWI was performed in additional 12 patients with clinically confirmed AS. The image results were analyzed by multiple post-processing techniques, as compared to conventional MRI. In AS patients, mean ADC values were (0.494 +/- A 0.170) x 10(-3) mm(2)/s in sacrum and (0.513 +/- A 0.129) x 10(-3) mm(2)/s in ilium, which were significantly higher than those of LBP ((0.306 +/- A 0.057) x 10(-3) mm(2)/s in sacrum and (0.323 +/- A 0.083) x 10(-3) mm(2)/s in ilium) and healthy volunteers ((0.315 +/- A 0.009) x 10(-3) mm(2)/s in sacrum and (0.319 +/- A 0.012) x 10(-3) mm(2)/s in ilium). No statistical differences were found between simple LBP and healthy volunteers. Mean ADC value of multiple lesions in AS was (0.932 +/- A 0.299) x 10(-3) mm(2)/s. By WB-DWI, abnormal signals of sacroiliac joints and extra-sacroiliac joint lesions were demonstrated in the locations corresponding with clinical findings. Mean ADC values of multiple lesions were (1.298 +/- A 0.323) x 10(-3) mm(2)/s in sacrum and (1.216 +/- A 0.311) x 10(-3) mm(2)/s in ilium. DWI and WB-DWI were shown to be effective in differentiation and systemic evaluation of early AS. Both techniques are likely to play an importance role in the early diagnosis of AS and assessment of treatment response.