Background/aim: Increased airway resistance reduces the effectiveness of ventilation treatment. Endotracheal tubes (ETTs) and connectors contribute to resistance. However, the effect of a closed system suction (CSS) connector is not well known. We compared the in vivo resistance occurring with a CSS connector with that of the standard connector. Materials and methods: This prospective study was conducted at Gazi University Hospital's neonatal intensive care unit. Intubated neonates were studied for two cycles; each cycle contained two periods of ETT + connector pairs (15 min/period) as follows: cycle 1 [A: long ETT + standard connector; B: long ETT + CSS connector] and cycle 2 [C: shortened ETT + standard connector; D: shortened ETT + CSS connector]. Resistance of 40 breaths/period was averaged for each case, and the means were analyzed by Wilcoxon test for pairwise comparisons between standard and CSS connectors. As each case provided two cycle data, 16 cycle data were compared. Results: The CSS connector increased resistance by 13.8% (range: 3.0%-22.1%) compared to the standard connector; P < 0.001. The resistance increase was similar between long [17.3% (range: 3.0%-17.7%)] and shortened ETTs [15.3% (range: 5.0%-29.6%)]; P = 0.834. Conclusion: CSS connectors were found to increase airway resistance in ventilated neonates. The contribution of CSS should be considered during ventilation, particularly in the presence of difficulty in providing sufficient tidal volume.