The effects of olanzapine and haloperidol on metabolic parameters in bipolar patients have been evaluated much less comprehensively than in schizophrenic patients. Therefore, in this study, medical records of 343 schizophrenic and bipolar patients treated with haloperidol or olanzapine for 1 year were retrospectively reviewed and metabolic outcomes were evaluated. After 12 months of follow-up, 25.9% of patients showed ≥3 metabolic abnormalities with a point prevalence of 27.2% in the bipolar and 24.9% in the schizophrenic group: 22.0% of the schizophrenic patients treated with haloperidol and 29.8% of those treated with olanzapine achieved ≥3 metabolic alterations; in bipolar patients, these percentages were 15.8% of those treated with haloperidol and 37.8% of those treated with olanzapine (p < 0.0001). Significant changes were reported over time in fasting glucose, triglycerides and cholesterol blood levels, systolic and diastolic blood pressure, body weight, and BMI. Overall, a significant number of schizophrenic and bipolar patients treated with olanzapine showed ≥3 metabolic alterations in the first month of treatment when compared to those treated with haloperidol. Moreover, the number of olanzapine-treated patients developing metabolic changes in the first month was significantly higher in both diagnostic groups when compared to those who reached metabolic abnormal values in the subsequent 11 months. These data suggest that both antipsychotics could increase the metabolic risk in schizophrenic and bipolar patients with a higher prevalence in olanzapine-treated patients. On the other hand, olanzapine-treated patients seem to achieve metabolic abnormalities faster than haloperidol-treated subjects in both diagnostic groups.