OBJECTIVE: The study aimed to identify patient characteristics associated with differences in outcomes of acute treatment in day hospitals and conventional hospital wards. METHODS: Data were from the European Day Hospital Evaluation (EDEN) (2000-2003), a multicenter randomized controlled trial comparing acute treatment in day hospitals and conventional wards in five European countries. All 880 adult patients in the EDEN study who had a diagnosis of schizophrenia, mood disorders, neurotic disorders, and behavioral syndromes (ICD-10 codes F20-F59) and complete data on baseline variables were included. Outcomes were symptom levels (Brief Psychiatric Rating Scale), subjective quality of life (Manchester Short Assessment of Quality of Life), and social disability (Groningen Social Disabilities Schedule) assessed at discharge and at three and 12 months postdischarge. Mixed- and main-effects models of interaction effects between patient characteristics and outcomes were generated. RESULTS: Patients' age, diagnostic category, and living status (alone or not) did not predict differences in outcomes between the two settings. However, patients with higher symptom levels at baseline experienced greater symptom improvements after treatment on a ward, and those with more years of education had greater symptom improvements after day hospital treatment. Female patients had more favorable social disability outcomes after day hospital treatment, but no difference between the two settings was found for male patients. CONCLUSIONS: More severe symptoms may be a reason for admission to acute treatment on a conventional ward rather than a day hospital. Female patients and those with more education may benefit more from acute treatment in day hospitals.