This is a prospective randomized (study medication is assigned by change) open-label, parallel-group, multicenter, 6 month study to compare the metabolic effects of paliperidone ER and olanzapine in patients with schizophrenia using the ratio of the concentration of lipids (triglycerides) in the blood to the concentration of good cholesterol (high density lipoproteins (HDL)) as the primary parameter. Secondary objectives include evaluation of additional parameters related to the total of the actions of the body to keep it alive (metabolic endpoints) and demonstration of non-inferiority of paliperidone ER versus olanzapine in efficacy as measured by Positive and Negative Syndrome Scale (PANSS). Patients previously treated with any oral antipsychotic, except those treated with paliperidone ER, olanzapine or clozapine during the last 6 months, can be enrolled and will be treated with paliperidone ER (6 to 9 mg/day) or olanzapine (10 to 15 mg/day). Patients will be divided into groups according to the metabolic effects of their previous antipsychotic medication (medication that does not increase body weight vs. medication that increases body weight). Throughout the study flexible dosing is allowed based on the investigator discretion. A study treatment period of 6 months is planned for all patients. Medication to treat symptoms like confusion, blurred vision, constipation, dry mouth, light-headedness, difficulty starting and continuing to urinate, and loss of bladder control may continue up to four weeks and should then be tapered off at the discretion of the investigator. Approximately 456 adult patients (228 in each treatment group) will participate in this study. Efficacy will be assessed with the following measures: PANSS (total score and subscale scores), Clinical Global Impression - Severity (CGI-S), Self-rated health status Survey SF-36, and Sleep and daytime drowsiness evaluation scale. The parameters related to the total of the actions of the body to keep it alive (metabolic endpoints) will be assessed with the following: ratio of blood lipids to blood good cholesterol concentrations (TG:HDL ratio) (for this primary evaluation, plasma fasting lipids and good cholesterol concentrations will be measured), fasting plasma insulin and fasting plasma glucose, plasma glucose and insulin concentrations before and after a 75 gram oral glucose tolerance test (OGTT) to asses insulin sensitivity and changes in insulin secretion, fasting good cholesterol, lipids, and glucose levels for the determination of new onset or presence of metabolic syndrome during treatment according to criteria of the Third Report of the National Cholesterol Education Program Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (NCEP/ATPIII criteria), weight, Body-Mass-Index and waist circumference for the determination of new onset or presence of a medical condition associated with abdominal obesity, abnormalities in glucose, lipid and cholesterol metabolism, and elevated blood pressure that increases the risk of cardiovascular disease and type 2 diabetes (metabolic syndrome) during treatment according to NCEP/ATP III criteria. All patients who receive trial medication (paliperidone ER or olanzapine) at least once will be included in the analysis of the demographic and baseline characteristic data. 2 dosage levels of paliperidone ER (6 or 9mg per day) and 2 of olanzapine (10 and 15mg per day) are available to the patients. Throughout the study flexible dosing is allowed based on the investigator's discretion. Study medication is to be taken in the morning orally, with water. A study treatment period of 6 months is planned for all patients.