Introduction: This study was conducted to describe the repeat multiparametric MRI (mpMRI)\nchanges occurring in prostate cancer (PCa) patients during active surveillance (AS), and to study\npossible associations between mpMRI-related parameters in predicting prostate biopsy (Bx)\nGleason score (GS) upgrading >3+3 (GU) and protocol-based treatment change (TC).\n\nMaterials and methods: The study cohort consisted of 76 AS patients with GS 3+3 PCa\nand at least two consecutive mpMRIs of the prostate performed between 2006 and 2015. Patients\nwere followed according to the Prostate Cancer Research International Active Surveillance (PRIAS) protocol and additional mpMRIs. The primary end points were GS upgrading (GU) (>3+3) in protocol-based biopsies and protocol-based TC.\n\nResults: 53\/76 (69%) patients had progression (PIRADS upgrade, size increase or new\nlesion[s]), 18\/76 (24%) had radiologically stable disease, and 5\/76 (7%) had regression\n(PIRADS or size decrease, disappearance of lesion[s]) in repeat mpMRIs during AS. PIRADS\nscores 4 to 5 in the initial mpMRI was associated with GU (p=0.008) and protocol-based TC\n(p=0.009). Tumour progression on repeat mpMRIs was associated with TC (p=0.045) but not with\nGU (p=1.00). PIRADS scores 4-5 predict GU (sensitivity 0.80 [95% confidence interval (CI);\n0.51-0.95, specificity 0.62 [95% CI; 0.52-0.77]) with PPV and NPV values of 0.34 (95% CI;\n0.21-0.55) and 0.93 (95% CI; 0.80-0.98), respectively.\n\nConclusion: mpMRI is a useful tool not only to select but also to monitor PCa\npatients on AS.