People with psychotic disorders experience to a great extent avoidable physical illnesses and early mortality. The aim of the study was to investigate the potential effects for this group of participating in a lifestyle intervention. A multi-component nurse-led lifestyle intervention using quasi-experimental design was performed. Changes in biomedical and clinical measurements, self-reported health, symptoms of illness and health behavior were investigated. Multilevel modeling was used to statistically test differences in changes over time. Statistically significant changes were found in physical activity, HbA1c and waist circumference. A lifestyle intervention for people with severe mental illness can be beneficial for increasing physical activity.