Background <p>Although hoarding disorder (HD) is commonly characterized as a chronic and progressive condition, relatively little is known about its longitudinal course and the factors that contribute to the progression, maintenance, or remission of hoarding symptoms. The current study, the first prospective investigation of hoarding symptom course, aims to identify and characterize patterns of hoarding symptom chronicity in a large sample of adults with clinically significant hoarding.</p> Methods <p>Participants who were enrolled in the internet-based Brain Health Registry completed online self-report measures of hoarding symptom severity at 6-month intervals over a period of 5 years. In a sample of 1,275 participants with clinically significant hoarding symptoms (79% with follow-up ≥ 4 years, <i>n</i> = 1,004), latent class mixed modeling (LCMM) was used to identify subgroups of individuals with similar symptom trajectories. Data-driven trajectories were then compared to four previously described theory-driven trajectories, and the demographic and clinical characteristics associated with trajectory class membership were assessed.</p> Results <p>LCMM yielded three distinct hoarding symptom trajectories: (1) persistent clinically severe symptoms (“severe”: <i>n</i> = 203, 15.9%), (2) persistent mild or subclinical symptoms (“mild”: <i>n</i> = 955, 74.9%), and (3) rapidly improving symptoms (“improving”: <i>n</i> = 117, 9.2%). Although this data-driven model characterized most participants as exhibiting a predominantly chronic course (90.8%), individuals in the “improving” class experienced sustained remission of hoarding symptoms within 18 months and were distinguished by later age of onset (median age = 47.5 years) and elevated prevalence of self-reported PTSD (20.2%). However, when integrated with the theory-driven framework, substantial heterogeneity in symptom progression emerged among those in the “mild” and “severe” classes, with 29% improving and 20% worsening over time. Individuals in the “severe” class with worsening symptoms (<i>n</i> = 41) were uniquely characterized by childhood-onset hoarding symptoms (median age = 13 years) and elevated prevalence of self-reported ADHD (35.0%).</p> Conclusion <p>Individuals with clinically relevant hoarding exhibit marked variability in longitudinal symptom course, including a distinct subgroup characterized by rapid improvement; these individuals had unique clinical and demographic characteristics compared to the other groups. These findings underscore the importance of monitoring intraindividual change and may inform early intervention and prevention strategies.</p> Clinical trial number <p>Not applicable.</p>