<qa>  What is the ICD-10 criteria for schizophrenia?Participants: Schizophrenic men and women between the ages 18 to 55 who meet the ICD-10 criteria for schizophrenia living in the catchment area of the psychiatric departments of Bispebjerg University Hospital,Rigshospitalet, or Psychiatric Center, Glostrup. Patients can be either unmedicated, or need to be switched from other antipsychotic medications due to side-effects, or lack of effect on negative symptoms, positive symptoms, or cognitive function. Patients can be enrolled in the study as inpatients or outpatients, and changes in hospitalization status during the trial are allowed. Patients were stabilized on antipsychotic medication ziprasidone before they were randomized to treatment with either donepezil or placebo. 