Few studies have compared the effect of Metformin treatment on metabolic parameters in atypical antipsychotic-treated schizophrenic patients. The present study examined metabolic abnormalities due to clozapine or olanzapine in schizophrenic patients and, secondly, the effect of metformin treatment on these parameters. Twenty-six patients (19 M and 7 F) from neuropsychiatric nursing home “Villa dei pini” (Avellino) were enrolled, in collaboration with our Department of Psychiatry, University of Naples SUN. All patients had been diagnosed according to DSM-IV criteria. They were recruited from July 2013 to January 2015. Clozapine or olanzapine-related hyperglycemia required metformin introduction in therapy. All prescribed drugs were maintained at the same therapeutic daily dose during our study. We performed 1, 3 and 6 months follow up after metformin initiation. For each patient fasting cholesterol, glucose, triglycerides, body weight (BW), body mass index (BMI), systolic and diastolic blood pressure (BP) were evaluated. SPSS 16.0 (Statistical Package for Social Science) was used for data analysis. After antipsychotic treatment BMI, fasting glucose and triglycerides were significantly higher respect to basal values (P < 0.01, P < 0.0001, P < 0.05, respectively). After metformin treatment, a significant improvement in fasting glucose, cholesterol and triglycerides was registered (P < 0.001). Conversely, BMI values, although not significant (P < 0.168), showed a trend in increasing. This observational study underlines that metformin in antipsychotic-treated patients could be useful in preventing clozapine or olanzapine related metabolic abnormalities in schizophrenic patients.