Aim To analyse the correlations between clinical and MRI sacroiliitis activity indexes in ankylosing spondylitis (AS). Materials and methods Sixteen normal volunteers and 52 patients were enrolled. The clinical AS activity indexes included the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) score, serum high-sensitivity C-reactive protein, and erythrocyte sedimentation rate. The MRI sacroiliitis activity indexes included apparent diffusion coefficient (ADC) values of bone and the Spondyloarthritis Research Consortium of Canada (SPARCC) score. Results The mean SPARCC score of the control group was 0 and of the AS group was 9.9 ± 9.1 (range 0–36). The mean ADC value of the AS group was statistically significantly higher than that of the control group (49.7 ± 20.2 × 10−5mm2/s versus 38.9 ± 5.3 × 10−5mm2/s, p < 0.05). The BASDAI score showed a statistically significant correlation with the SPARCC score (r = 0.685, p < 0.05) and with ADC values (r = 0.329, p < 0.05). ADC values correlated with the SPARCC score (r = 0.390, p < 0.05). Conclusion ADC values and the SPARCC index may be useful activity indexes in AS.