Many published studies underlined the relationship between Atypical Antipsychotic treatment and Metabolic Syndrome (MS) onset, but only few have compared the effects of Typical and Atypical Antipsychotics to this respect. The present study examined in schizophrenic and bipolar patients the impact of haloperidol (mean dose 3.37±2.28 mg and 2.51±1.48 mg, respectively) and olanzapine treatment (mean dose 12.42±6.53 mg and 9.97±5.36 mg, respectively) on glycemia, blood pressure, BMI, triglycerides and HDL cholesterol. A higher prevalence and a different time of appearance of Metabolic Syndrome was observed in Bipolar patients when treated with olanzapine. Moreover, weight increase was greater in bipolar patients treated with olanzapine (p< .000) when compared to schizophrenic treated with both Antipsychotics. These findings suggest a shared susceptibility to antipsychotic- related metabolic disregulations in both schizophrenic and bipolar patients for SGAs, but not for FGAs. Moreover, the effects of olanzapine in bipolar patients support the development and testing of interventions specifically designed for preventing and treating the metabolic syndrome and its components.