Background: The aim of the study was to determine the costs of treating schizophrenia from the perspective of the statutory health insurance, as well as the identification of predictors of hospitalisation of formerly stable schizophrenia patients. Methods: Claims data for the years 2004-2006 were analysed. Patients who did not have to be treated in a hospital as a result of an ICD diagnosis F20 both in the year 2005 as well as also in 2006 were defined as stable patients. In contrast, those patients who had to be treated in a hospital in 2006 because of a diagnosis of schizophrenia were defined as unstable. In addition to the overall healthcare costs, the costs specific to schizophrenia were also analysed. Also, based on binary logistic regression analysis, predictors for hospital treatment were determined. Results: 8497 stable and 1449 unstable patients were identified. The schizophrenia specific costs for stable patients were € 1605 and the overall costs were €4029 in 2006, respectively. Unstable patients had indication-specific costs amounting to € 12864 and overall health care costs of € 16824. For unstable patients, the costs of hospital treatment were identified as being a substantial cost area. Predictors for a higher probability of hospital treatment were: female patients, at least one rehabilitation measure, at least one stay in hospital in 2004, and being co-morbid with substance abuse. In contrast, older patients, who were treated with concomitant medications, and if they received a continuous drug therapy in all quarters of a year had a lower probability of hospitalisation. In addition, an increased number of visits to a doctor reduced the probability of hospitalisation. The variable 'depot medication' were close to significance and the variable inability to work lasting more than six weeks' had, in contrast, no significant influence.Conclusions: The schizophrenia specific and overall health care costs of unstable patients were clearly higher than was the case with stable patients and mainly determined by inpatient hospital treatment. A range of potential predicting factors which can be extracted from routine claims data have a positive or negative influence on the probability of treatment in hospital.