In addition to killing pathogens and influencing immunological processes, cathelicidin LL-37 is a multifunctional host defense peptide with a role in homeostasis. It has been suggested that imbalances in homeostatic signaling from inflammatory/ immune, endocrine and metabolic cascades and oxidative stress may partially contribute to the pathogenesis of mental disorders. The purpose of the study was to identify any differences in LL-37 levels between patients with schizophrenia, euthymic bipolar disorder, bacterial pneumonia, pulmonary tuberculosis, and healthy controls. Thirty-five patients with chronic paranoid schizophrenia, 40 patients with chronic, euthymic bipolar disorder, 30 patients with bacterial pneumonia, 32 patients with pulmonary tuberculosis, and 38 healthy volunteers were included in this study. The patients with schizophrenia demonstrated a significantly lower level of LL-37 than those with bipolar disorder (p=0.006) and those with pulmonary TB (p less than 0.001). Significant differences in LL-37 levels were found between patients with bipolar disorder, bacterial pneumonia (p less than 0.001) and pulmonary TB (p=0.004). Our findings suggest that changes observed in the serum level of LL-37 in psychiatric patients enrolled in this study could be a result of homeostatic imbalance.