Background With the advanced MRI techniques, pathologic features can be detected at an early stage and quantitatively evaluated, resulting in the advantages of early diagnosis and prompt treatment. This study aimed to determine the value of diffusion-weighted MR imaging (DWI) in detection of early ankylosing spondylitis (AS) and investigate the characteristic manifestations of AS on whole body DWI (WB-DWI). Methods Twenty patients with the diagnosis of early AS, twenty patients with low back pain (LBP), and twenty-five healthy volunteers were included in this study. The subchondral bone apparent diffusion coefficients (ADC) among these groups in the bilateral ilia and sacrum along the sacroiliac joints were compared. An independent sample t-test was utilized to analyze ADC value differences among groups. P-values less than 0.05 denoted statistical significance. The mean ADC values of focal DWI lesions in AS patients were also measured. Whole body diffusion-weighted imaging was performed in fifteen additional AS patients, and analyzed with MIP and MPR techniques in comparison to conventional MR images in order to evaluate the ability to detect AS lesions with whole body DWI. Results Mean ADC values in AS patients were (0.518 +/- 0.122)x10(-3) mm(2)/s in the ilium and (0.503 +/- 0.168)x10(-3) mm(2)/s in the sacrum. These were significantly greater than the values measured in the ilium and sacrum of LBP patients, (0.328 +/- 0.053)x10(-3) mm(2)/s in the ilium and (0.311 +/- 0.081)x10(-3) m(2)/s in the sacrum, and control group, (0.325 +/- 0.015)x10(-3) mm(2)/s in the ilium and (0.318 +/- 0.011)x10(-3) mm(2)/s in the sacrum respectively. No statistically significant differences were found between LBP group and control group. The mean ADC value of focal DWI lesions in early AS patients was (0. 899 +/- 0.265)x10(-3) mm(2)/s, which was significantly higher than that of adjacent normal-appearance areas ((0.454 +/- 0.079)x10(-3) mm(2)/s). WB-DWI detected abnormalities in the 15 additional AS patients both within the sacroiliac joints and at other sites, corresponding to the clinical symptoms of the patients. The mean ADC value of focal DWI lesions of this patient cohort was (1.286 +/- 0.311)x10(-3) mm(2)/s in the sacrum and (1.220 +/- 0.299) x10(-3) mm(2)/s in the ilium. Conclusions Subchondral marrow ADC values of subchondral marrows near the sacroiliac joints allow for the differentiation of patients with early AS from normal volunteers and LBP patients. Combined with post-processing techniques such as MIP and MPR, WB-DWI allows for the comprehensive assessment of AS patients, an evaluation potentially helpful in determining prognosis and following the therapeutic response.