Glycine is a non-competitive NMDA co-agonist of an ionotropic NMDA receptor in the glutamate system. Beneficial effects of this amino acid on primary negative symptoms of schizophrenia were revealed in the literature. The main research hypothesis assumes the existence of a relationship between negative symptoms of schizophrenia and serum glycine concentration. Aim. The aim of the study is finding a relationship between the initial glycine serum concentrations together with changes in concentration of this compound and severity of schizophrenia symptoms (in PANSS) and selected cognitive functions as a result of application of glycine at a dose of 0.8g/kg/day orally added to the existing antipsychotic treatment. Method. A group of 28 individuals with schizophrenia with predominantly negative symptoms completed a 6-week open-label prospective study. The patients received oral glycine (60g/day) in parallel with the existing antipsychotic treatment At the beginning and end of the study serum glycine levels were measured and the severity of the symptoms of schizophrenia using PANSS and cognitive functioning (using Wisconsin Card Sorting Test, Trail Making Test and Stroop Test) were assessed. Results. From the data, obtained on two check-ups before and after the 6 weeks of glycine application, no significant correlations between serum glycine concentrations and either the PANSS score (subscales assessing positive and negative symptoms, general psychopathology and a total score) or the cognitive battery results were found excluding a correlation between the increase in the glycine concentration and the reduction in the number of errors in the second part of the Trail Making Test (TMT). Conclusion. Measurements of serum amino acid concentrations do not allow direct assessment of severity of disease symptoms or resulting improvement. Unfortunately, the initial serum glycine concentration may not be used as a predictor of the improvement resulting from augmentation of antipsychotic treatment with glycine. The results obtained in the whole PANSS scale and in its subscales suggest beneficial effects of glycine in the applied dose on the psychopathology of schizophrenia.