Background <p>The longitudinal course of hoarding behavior (HB) in children and adolescents, as well as the predictors of its outcome, remain unknown. This prospective longitudinal study aims to evaluate the four-year course of HB and identify predictors of progression to hoarding disorder (HD) and the persistence of hoarding symptoms.</p> Methods <p>Children and adolescents with HB underwent a clinical assessment after a four-year follow-up period. The assessment included a one-on-one psychiatric interview, an HD clinical interview based on DSM-5 diagnostic criteria, Schedule for Affective Disorders and Schizophrenia for School-Age Children-Present and Lifetime version (K-SADS-PL), a semi-structured diagnostic interview for comorbid diagnoses, and the Children’s Saving Inventory (CSI) to assess hoarding symptom severity.</p> Results <p>Over the 4-year follow-up period, 42.2% (<i>n</i> = 19) of participants progressed to HD, 20% (<i>n</i> = 9) persisted in HB, and 37.8% (<i>n</i> = 17) no longer displayed hoarding symptoms. Baseline hoarding symptom severity significantly predicted progression to HD and was also associated with persistence of hoarding symptoms. In exploratory models, current psychiatric comorbidity and female gender were associated with persistence but not progression.</p> Conclusions <p>These findings provide precious insights into the longitudinal course of HB and predictors. The results also suggest the importance of early identification and monitoring of HB.</p>