The objectives of this prospective clinical study are to quantify the variation of pressure exerted by the upper lip on the upper teeth during the alignment phase and to evaluate its capacity to adapt to changes in dental position.Thirty young subjects in skeletal Class I relationship requiring non-extraction orthodontic treatment were included in this study. The pressure exerted by the upper lip on the upper central incisors and right canine were measured during rest and swallowing positions using a pressure transducer before bracket placement (T0), after bracket placement (T1), three months (T2) and six months later (T3). Maxillary intercanine width (CC), upper arch length (U) and crowding (C) were measured on stone models at T1, T2 and T3 to determine the existence of a correlation between the variation of lip pressure and these variables.The lip pressure significantly increased after bracket placement and remained relatively stable during the six-month period. The labial pressure on the incisors was the only variable to significantly decrease at T3, though remaining significantly higher than the starting pressure. A positive correlation was found between the variation of the inter-canine distance and the labial pressure on the canine at rest whereas a negative correlation exists between the crowding and the labial pressure on the incisors at rest.This study showed that increasing inter-canine width disrupts the muscle equilibrium and therefore is prone to relapse, whereas the upper lip can better adapt to the protrusion of upper incisors.