The goal of the present study is to evaluate the effect of D-serine, added to antipsychotic treatment, on negative and cognitive symptoms in schizophrenia.

The investigators are hypothesizing that D-serine will improve cognitive functioning and negative symptoms.

Inclusion Criteria:

• Diagnostic and Statistical Manual of Mental Disorders - Fourth Edition (DSM-IV) diagnosis of schizophrenia and\or schizoaffective disorder.
• Age between 18 to 64.
• PANSS negative symptom score higher than 19.
• SAS total score lower than 12.
• CDSS suicidal risk lower than 2

Exclusion Criteria:

• Epilepsy
• Meets DSM-IV criteria for mental retardation.
• Medical condition which confounds presentation or treatment (e.g. uncontrolled diabetes mellitus, history of kidney stones, acute drug toxicity, dementia, delirium, stroke, etc.)
• Meets DSM-IV criteria for alcohol or drug dependence in last 6 months.
• Meets DSM-IV criteria for alcohol or drug abuse in last one month.
• Treatment with clozapine.
• Current positive pregnancy test or not using acceptable method of birth control.
• Meets DSM-IV criteria for current anxiety or mood disorders