Glutamate dysregulation may be involved in the neuropathology of schizophrenia. Memantine, a drug approved by the FDA for the treatment of moderate to severe Alzheimerʹs disease, acts as a partial uncompetitive NMDA receptor antagonist. The aim of this study was to examine the efficacy of memantine as an adjunctive treatment to olanzapine in patients with schizophrenia. In this double-blind, placebo-controlled studies, patients with schizophrenia according to DSM-IV clinical criteria were selected. Patients were randomly assigned to receive either memantine (week 1:10 mg/day; weeks 2-6:20 mg/day) plus olanzapine (15-20 mg/day) or olanzapine plus placebo. At baseline, no statistically significant difference regarding the mean total PANSS scores between treatment groups was found. Results showed that memantine significantly improved the positive and negative PANSS score in patients maintained on olanzapine after six weeks compared to olanzapine alone (P<0.001). Furthermore, female patients showed significantly better response than males, especially in positive PANSS score. No significant changes in extrapyramidal symptoms were observed.These findings indicate that olanzapine efficacy might be augmented with memantine. Furthermore, this effect is more remarkable in female patients with schizophrenia.