N-Methyl D-aspartate (NMDA)-receptor hypofunction has been implicated in the pathophysiology of schizophrenia and D-serine and glycine add-on therapy to antipsychotics has shown beneficial effects in schizophrenic patients. Nevertheless, previous studies have not shown consistently altered D-serine concentrations in cerebrospinal fluid (CSF) of schizophrenic patients. To confirm and extend these results, CSF concentrations of both endogenous NMDA-receptor co-agonists d-serine and glycine and their common precursor L-serine were analyzed simultaneously in 17 healthy controls and 19 schizophrenic patients before and 6 weeks after daily olanzapine (10 mg) treatment. CSF D-serine, L-serine and glycine concentrations and their relative ratios were similar between schizophrenic patients and controls and no differences were observed before and after olanzapine therapy. Thus, the NMDA-receptor hypofunction hypothesis in schizophrenia is not explained by olanzapine therapy-dependent absolute or relative decreases in CSF D-serine and glycine concentrations in this series of male patients, thereby not providing convenient markers for the disorder.