<b><i>Background:</i></b> Schizophrenia (SCZ) is a chronic severe neuropsychiatric disorder, where pharmacological treatment has been hindered by adverse effects, including antipsychotic-induced weight gain (AIWG) and related complications. Genetic studies have been exploring the appetite regulation and energy homeostasis pathways in AIWG with some promising leads. The serotonin system has been shown to participate in these pathways. <b><i>Methods:</i></b> In the current study, we examined single nucleotide polymorphisms across the serotonin receptor genes <i>HTR3A</i> and <i>HTR3B</i>. Prospective weight change was assessed for a total of 149 SCZ patients of European ancestry. <b><i>Results:</i></b> We did not find the tested <i>HTR3A</i> or <i>HTR3B</i> gene markers to be associated with AIWG in our sample. <b><i>Conclusion:</i></b> Our preliminary findings suggest that these receptors may not play a major role in predicting AIWG.