The invention relates to an adjustment device for chairs, for adjusting a swaying resistance for a backseat part of a chair. The adjustment device comprises a chair base (10), a backrest part (20) pivotally arranged to the chair base (10) around a pivot shaft (40) and at least one spring element (31, 32) having a first end (31 a, 32a) arranged to the chair base (10) and a second end arranged to the backrest part (20), whereby the spring element (31, 32) has a spring force having a force direction parallel to the direction of the spring element from its first end to its second end, and that the spring element is arranged such that when the backrest part is loaded, the spring element will be loaded. The adjustment device further has a setting mechanism (50, 51) for setting the force of the spring element, whereby the setting mechanism (50, 51) is arranged to move the spring element (31, 32) such that the force direction of the spring element is changed and whereby the setting mechanism (50, 51) is arranged to move the first end (31 a, 32a) of the spring element or the second end (31 b, 32b) of the spring element substantially perpendicular to the force direction of the spring element. The invention further relates to a chair provided with such an adjustment device.