The aim of this study was to analyze the changes in dental arch shape during adolescence and to assess its relation with late mandibular crowding. The sample included 68 (49% female) subjects, orthodontically untreated Caucasians, from the Nittedal growth study in Norway. They were recalled for check-ups every three years. This study analyses the data for the ages of 12, 15, 18 and 21. Measurements included anterior arch depth, intercanine, interpremolar/anterior and intermolar/posterior width, Little's and Bolton's index and the ratio between anterior arch depth and width. The intercanine width in men decreasees between the age of 12 and 15 and grows from the age of 18 to 21 (p=0.006). The posterior width decreases between 12 and 18 years and increases after the age of 18. Men have significantly higher posterior widths than women at any age (p<0.05). With the increase of age, the anterior arch length decreases (p<0.05). Mandibular incisor crowding increases during adolescence (p<0.05). Logistic regression demonstrated that intercanine width is the only occlusal factor contributing to crowding. Increase of intercanine width in period 12-21 years of age for >1mm reduces the risk for manidbular incisor crowding for >1mm in the same period by 74% (OR: 0.265: 95% CI 0.076-0.931; p=0.045). Dental arches continue to change during adolescence. The shape of mandibular dental arch becomes more square-shaped, mostly due to decrease of the mandibular arch length. Mandibular incisor crowding does occur during adolescence and the increase in mandibular intercanine width reduces the risk for its development