Mental illness is often accompanied by poor physical health and shorter life expectancy. Second-generation antipsychotics (SGAs) are suspected of increasing cardiovascular risk, possibly through development of metabolic syndrome (MetS), and the risk of adverse outcome is even higher if obesity or metabolic aberration starts in childhood or adolescence. Drug-naive adolescents were recruited after contact with an outpatient Psychosis Team. Changes relative to baseline in body mass index (BMI), waist circumference (WC), blood pressure (BP), fasting blood glucose (FBG), triglycerides (TG), and high-density lipoprotein (HDL) cholesterol were determined through regular follow-ups. The sample included 35 SGA-naive patients aged 7-19 (mean: 15.5) with a diagnosis of psychosis. Over 12 months, the overall rate of MetS changed significantly (from 0% to 20% [p < 0.016]). There was a significant increase in BMI (18.4% [p < 0.001]), WC (14.3% [p < 0.001]), TG (25.2% [p = 0.039]), and FBG (3.6% [p = 0.038]), whereas there was a significant decrease in HDL (-11.5% [p < 0.001]). No significant change was found for BP. Compared with the 2014 Danish references BMI-for-age charts, after 12 months the participants' BMI had increased from 0.5 to 1.57 standard deviation (SD) above the 50th percentile for age and gender (p = 0.0001). To our knowledge, this is the first study to include all the aspects of MetS in a sample of drug-naive adolescents followed over the first 12 months after starting SGA treatment. A significant shift in all parameters (except BP) toward MetS was found, presumably due to SGA use. Therefore, these adolescents will need proper follow-up, consisting of not only monitoring but also preventive measures to diminish these effects of SGA use.