Background: A medication use review (MUR) aims to optimize medication use, patient knowledge and can improve health outcomes. This pharmaceutical care service is not yet available in Belgium. Objectives: To describe drug-related problems (DRPs) detected during a MUR, subsequent interventions proposed by pharmacists and evolution of DRPs until follow-up and to identify patient-related variables associated with the number of reported DRPs. Methods: Belgian community pharmacists provided a MUR to older polymedicated ambulatory patients and registered DRPs, interventions and resolution at follow-up using the PharmDISC classification. The relationship between 14 patient-related variables and the number of reported DRPs was investigated with univariate analysis. A prediction model was developed with significant variables using negative binomial regression analysis. Results: Across 56 pharmacies, 453 patients received a MUR and 1196 DRPs were registered (median 3DRPs/ patient, range 0–10). Only for 11.7% of patients no problems were identified. The top-3 causes were interaction (15.2%), inappropriate timing or frequency (13.5%) and adverse effect (11.9%). The top-3 recommended interventions by pharmacists were transmission of information (25.1%), in-depth patient counselling (15.0%) and therapy stop (8.2%). After six weeks, 42.6% of DRPs were resolved; data was missing for 33.3%. A higher number of chronic drugs, female gender and living alone were associated with more DRPs. The prediction model found that per additional chronic drug, the number of problems increases by 4.3% (95% CI: 2.0–6.6%). Male gender decreases DRPs by 22.1% (95% CI: 10.4–32.0%). Living alone provided no additional predictive value in the prediction model. Confounding process- and pharmacist-related variables also influenced the number of reported DRPs. Conclusion: A MUR appears an effective strategy to detect and resolve DRPs. The number of chronic medications and female gender predict a higher number of DRPs. These findings are a starting point for evidence-based eligibility criteria for a MUR service in Belgium.