Diagnostic comorbidity is common among psychiatric patients. One approach to obtain an improved understanding of this phenomenon is to explore diagnostic transitions, across time. In the present study, we aim to take this methodology further within the specific context of eating disorders by investigating whether there are individuals who shift not only between different eating disorder symptoms, but also shift to deliberate self-harm and substance use over time, and relations of this to other indicators of mental illness. Retrospective longitudinal registry data from the Swedish Stepwise national quality assurance platform for eating disorders were analyzed, including self-report measurements and clinical assessments. Individuals (N = 3,159 adults) were selected for analysis based on available data at admission and 12 months postadmission, and grouped based on occurrence of a "symptom shift" (defined as a decrease in one symptom and increase in another over time). In this sample of patients, 422 (13%) demonstrated symptom shifting among eating disorder symptoms. As hypothesized, "symptom shifters" were more prone to engage in deliberate self-harm and shifted to both deliberate self-harm and substance use across time. They had higher reported levels of symptoms indicative of mental illness (e.g., anxiety and compulsivity) and more pronounced functional impairment (clinician rated and self-rated), compared to nonshifters. Taken together, this study demonstrates that a subgroup of individuals diagnosed with eating disorder(s) shift between distinct psychiatric symptoms across time, indicating that they may share a common vulnerability to engaging in problem behaviors and a need for a more comprehensive and individualized treatment plan. (PsycINFO Database Record (c) 2019 APA, all rights reserved).