Objectives: The primary aim is to evaluate signs of inflammation on MRI of sacroiliac joints (SIJ)/spine in inflammatory back pain (IBP) patients suspected of nr-axSpA with high disease activity. Secondary aims are to describe the onset of new inflammatory lesions at MRI after 6 months and to evaluate gender differences in the presence of inflammation. Method: Consecutively, patients with IBP with at least two spondyloarthritis features, high disease activity (BASDAI ≥ 4), and who were TNFi naïve, had a MRI of SIJ and spine. In the absence of active lesions, MRI was repeated after 6 months. MRI images were scored according to the Spondyloarthritis Research Consortium of Canada method. Results: Sixty-nine patients were included (53% female), of whom 39% showed signs of inflammation at the first MRI: 30.9% of the SIJ, 19.1% of the spine and 2.4% at both sites, irrespective of the CRP levels. Males more often showed inflammatory signs at the MRI of the SIJ and spine compared with females (45.5% vs. 33.3%). Consistently, the median SPARCC score was higher in males: for SIJ 14.0 (IQR 2.3–25.0) and for spine 11.5 (IQR 8.5–25.6). Only one patient (4.7%) without baseline inflammatory signs showed active lesions of SIJ after 6 months. Conclusions: Almost 40% of the IBP patients suspected of nr-axSpA, with high disease activity, showed inflammatory lesions on MRI of SIJ and/or spine, which occurred more often in males compared with females. In the majority (95.3%), an MRI without inflammatory lesions remained negative after 6 months despite high disease activity.Key Points• Forty percent of inflammatory back pain patients with high disease activity showed inflammatory signs on MRI of the SIJ and/or spine.• Only 4% of baseline MRIs without inflammatory signs at baseline conversed to an MRI with inflammatory signs after 6 months.• Male inflammatory back pain patients with high disease activity showed more often inflammatory signs on MRI compared with females.