Abstract Glycine acts as an endogenous selective co-agonist at the glycine modulatory site of the NMDA ( N-methyl- d-aspartate) receptor. Significantly decreased glycine serum levels were reported in patients with schizophrenia in comparison to healthy controls. Administration of glycine improved negative symptoms in patients with schizophrenia treated with antipsychotics in some clinical trials. We hypothesized that glycine serum levels might be associated with intensity of negative symptoms in schizophrenia. Fifty outpatients with the diagnosis of schizophrenia as defined by ICD-10 and fifty age- and gender-matched healthy controls were recruited into the study. Glycine serum levels were measured by high performance liquid chromatography (HPLC). We used the Positive and Negative Syndrome Scale (PANSS) and the Scale for the Assessment of Negative Symptoms (SANS) to assess the symptoms of schizophrenia in the patients. We found mean glycine serum levels to be significantly lower in patients than in controls. This difference was only caused by findings in the male study population. Glycine serum levels were negatively associated with intensity of negative symptoms assessed by the PANSS negative subscale and the SANS total scores in the patients. These data suggest a possible implication of NMDA receptor dysfunction in the pathogenesis of negative symptoms in schizophrenia.