Urotensin-II (UT-II) is the most powerful vasoconstrictor agent and is known to play a role in heart failure, diabetes, pulmonary hypertension and asthma. The effect of passive smoking on UT-II levels is unknown. The present study aims to evaluate serum UT-II levels in children exposed to passive smoke. The study included a total of 120 children; 47 children not exposed to passive smoke were included in Group 1 (control group), and 73 children exposed to passive smoke were included in Group 2. Serum samples of the participants were stored at-80 °C after centrifugation and were assessed at least two times with high-precision human ELISA kits. Serum UT-II levels were significantly higher in the children exposed to passive smoke than in the children not exposed. Furthermore, Group 2 was grouped according to the number of cigarettes smoked at home per day, type of passive smoking (second-hand smoke or third-hand smoke), and how many people in their family and/or living together smoked. There was a positive correlation between the number of cigarettes they were exposed to per day and serum UT-II levels. Passive smoking in childhood may be associated with high serum UT-II levels.