OBJECTIVES: To provide a study description and baseline socio-demographic, socio-economic and clinical characteristics of schizophrenia patients treated with haloperidol, olanzapine or risperidone at time of enrolment in the SOS observational study. METHODS: SOS is a non-randomised 2-year prospective observational study. Data are collected on treatment costs, efficacy, tolerability and patient quality of life using an internet-based case report form on patients assigned treatment with haloperidol, olanzapine or risperidone during a recent psychotic episode requiring hospitalisation. Patients were included within one month from discharge to part-time hospitalisation or ambulatory care if treatment with the study drug was stable since at least a month. Data are collected at inclusion and after 3, 6, 12, 18 and 24 months. RESULTS: 323 patients diagnosed with schizophrenia or schizofreniform disorders (DSM-IV criteria) have been enrolled in the study. Preliminary baseline data on 305 patients (males: 65%) show that 32 patients are on haloperidol, 141 on olanzapine and 132 on risperidone. 77% are outpatients. 57% suffer paranoid schizophrenia. Mean age is 37±13y. The socio-economic profile (living environment, schooling background and working activity) is comparable between treatment groups. "Brief Psychiatric Rating Scale" and "Clinical Global Impression" scores (overall mean: 32±16 and 4.0±1.2 respectively) are comparable for all groups. However, some variables such as first antipsychotic prescription (haloperidol: 6.3%; olanzapine: 18%; risperidone: 24%) suggest that there may be differences between the treatment groups. "Global Assessment of Functioning" score (mean: 52±14) is significantly lower in the haloperidol group (47±12). General quality of life (EQ-5D) is comparable for all groups (66±19). "Subjective Well-Being under Neuroleptics" scores are significantly worse in the olanzapine group. CONCLUSION: The SOS observational study will provide substantial information to increase the understanding of the clinical, social and economic outcomes and current treatment modalities in real world clinical practice in the treatment of schizophrenia in Belgium.