We aim to investigate whether the -759C÷T polymorphism in 5-HTR2C gene was associated with weight change and hyperinsulinemia in Romanian pediatric patients with schizophrenia and bipolar disorders. The patients under investigation were enrolled between 2009 and 2014. A total of 81 schizophrenic and bipolar-disorder patients, aged between nine to 20 years (median age 15.74±4 years), who were following an atypical antipsychotic treatment (Risperidone, Aripiprazole, Olanzapine), were enrolled from University Hospital for Child and Adolescent Psychiatry and Neurology from Timisoara, Romania. The outcomes that we measured were the changes in Body Mass Index (BMI) from baseline to different time points: three months, six months, 12 months and 18 months, and the change in insulinemia over time, after atypical antipsychotic treatment. After carrying out the 5-HTR2C 759C÷T polymorphism identification, we found that 22 patients presented the -759C÷T polymorphism in 5-HTR2C gene. Between the patients exhibiting the 5-HTR2C -759C÷T polymorphism and the patients having the wild type alleles, there was no significant statistical difference in changes of BMI from baseline to endpoints that indicates the lack of the protective effect of the T allele against atypical antipsychotics-induced weight gain. Interestingly, we found a statistically significant association between insulinemia and T alleles' carriers, after 18 months of treatment with the above-mentioned antipsychotics. Taking into consideration that atypical antipsychotics have been associated with elevated insulin levels and insulin resistance, maybe in the future the -759C÷T polymorphism would find a role in the development of a more complex algorithm for prediction of diabetes mellitus risk, in patients taking atypical antipsychotics.