Despite initial data suggesting positive treatment outcomes for adolescent eating disorder day-hospital programmes (DHPs), existing studies have included limited follow-up, small samples, and a focus on restricting-type eating disorders. To address these gaps, we explored naturalistic outcomes for an adolescent eating disorders DHP. Adolescent participants (N = 265) completed measurements at treatment admission, discharge (n = 170), and various lengths of follow-up (n = 126; M = 278.87 days). Results from multilevel models indicated significant increases in body weight for the anorexia nervosa group throughout treatment and maintenance of increased body weight from discharge to follow-up. In bulimic spectrum disorders, binge eating and purging significantly decreased from intake to discharge and did not change from discharge to follow-up. Across the entire sample, eating disorder symptoms decreased from intake to discharge and did not change from discharge to follow-up. Further, anxiety and depression decreased over the course of treatment and continued to decrease over the follow-up period. The current investigation represents the first study to explore longitudinal DHP outcomes within adolescent bulimic spectrum eating disorders. Our findings also highlight many challenges inherent in conducting naturalistic research; it is critical that the field continue to develop solutions to the barriers inherent in conducting longitudinal research on eating disorder treatment.