Aim of the present study is to compare magnetic resonance imaging (MRI) and positron emission tomography (PET) parameters in the follow up of chronic periaortitis (CP), with a focus on changes in the apparent diffusion coefficient (ADC) and standardized uptake values (SUV). 127 patients with CP were treated in our urology between 2007 and 2017. We identified 14 patients with parallel abdominal MRI and PET–CT examinations before therapy and in the follow up resulting in a total of 56 examinations. Relative contrast uptake and diffusion-weighted MRI parameters were compared to SUV in the corresponding PET–CT examinationsand laboratory infection markers. All examined MRI and PET–CT parameters showed significant changes between basis and follow-up examinations. Median ADC values increased significantly (p < 0.001) in the follow up. SUVmax and the other MR parameter (contrast uptake, DWI-signal) declined significantly. We observed a strong negative correlation between ADC and SUVmax (rho: − 0.61; p < 0.001). In addition, we found an inverse correlation of ADC with the inflammation markers ESR (rho: − 0.64; p < 0.001) and CRP (rho: − 0.54; p = 0.001). MRI and PET–CT showed comparable results in the individual follow up of CP and compared to laboratory inflammation markers. Our data support MRI as first imaging modality due to absent radiation and the necessity of repetitive follow-up examinations in patients with CP.