Polyploidization is a widespread method used in peppermint breeding to increase the chromosome numbers and to restore the fertility as a prerequisite for combination breeding and the induction of new genetic diversity. The use of these methods in breeding new varieties often results in progenies with increased chromosome numbers and characters divergent in comparison with the parents. Ten peppermint cultivars were classified into two groups according to their chromosome numbers: Group 1 with normal chromosome numbers 2 n = 66–72, Group 2 with increased chromosome numbers 2 n = 90–96. Cultivars with the higher chromosome numbers showed a more vigorous growth, the generative development was accelerated, the essential oil content and its content of menthone, isomenthone and limonene were increased and the content of menthol, menthofuran, 1,8-cineole and viridiflorol was lowered.