The aim of this observational study was to examine the short-term effectiveness of a structured, largely manualised day hospital treatment, as well as the stability of short-term effects in patients with anorexia nervosa (AN) and bulimia nervosa (BN). Eighty-three patients, 47 with AN and 36 with BN, were assessed at pre-treatment, at the end of the day hospital treatment and at follow-up approximately one year after post-treatment. Outcome measures were body mass index (BMI), disturbed eating attitudes and behaviours assessed by the Eating Disorder Inventory (EDI), frequency of bingeing and purging, and general psychopathology assessed by the Symptom Checklist-90 (SCL-90). At the end of the day hospital treatment, significant improvements could be found on all outcome variables (frequency of binge eating/vomiting/laxative abuse, BMI and core EDI-subscales 'drive for thinness'/'bulimia'/'body dissatisfaction'). Effect sizes were generally large (.74 > d < 2.44). The effects were maintained or improved until follow-up (18 months). At follow-up, 40.2% of AN patients and 40.4% of BN patients could be classified as remitted. General psychopathological impairment was also significantly reduced at post-treatment and maintained until follow-up. The results demonstrate both the short-term effectiveness and long-term stability of day hospital treatment in a large sample of patients with anorexia and BN.