Medical treatment chair, in particular for dialysis treatments, with a backrest and side armrests, - wherein the distance between the armrests in the transverse direction of the chair is continuously adjustable, so that the armrests can be swivelled about pivot axes near the chair back from a mutually approximately parallel starting position to an approach position, in which the ends of the armrests further from the back in the transverse direction of the chair are at a lesser distance from one another than the ends of the armrests near the backrests, - wherein the armrests can be coupled with a backrest adjustable between a sitting position and a reclining position, such that the armrests maintain their inclination relative to the seat plane substantially unchanged when the backrest is adjusted, - wherein the armrests can be fixed in any arbitrary orientation of their longitudinal axes relative to the backrest, characterised in that - the armrests are arranged pivotally on support arms about axes perpendicular to their support surface, wherein the respective pivot position of the armrests relative to the support arms can be fixed by means of a clamping wheel, wherein the support arms in turn are pivotally arranged on the frame of the backrest, wherein the respective pivot position can be locked by actuating a locking device and a transverse axis of the support arms and the pivot bearing of the backrest are coupled to one another by a gear mechanism installed in the housing-like frame of the backrest in such a way that the support arms are pivoted synchronously when the backrest is swivelled between the sitting position and reclining position and the respective inclination of the supporting arms relative to the seat level remains unchanged, - the transverse axes of the support arms are axially displaceably arranged within the frame of the backrest so that the distance between the support arms can be changed or adjusted in the transverse direction of the seat. - wherein the distance between the armrests in the transverse direction of the chair is continuously adjustable, so that the armrests can be swivelled about pivot axes near the chair back from a mutually approximately parallel starting position to an approach position, in which the ends of the armrests further from the back in the transverse direction of the chair are at a lesser distance from one another than the ends of the armrests near the backrests, - wherein the armrests can be coupled with a backrest adjustable between a sitting position and a reclining position, such that the armrests maintain their inclination relative to the seat plane substantially unchanged when the backrest is adjusted, - wherein the armrests can be fixed in any arbitrary orientation of their longitudinal axes relative to the backrest, characterised in that - the armrests are arranged pivotally on support arms about axes perpendicular to their support surface, wherein the respective pivot position of the armrests relative to the support arms can be fixed by means of a clamping wheel, wherein the support arms in turn are pivotally arranged on the frame of the backrest, wherein the respective pivot position can be locked by actuating a locking device and a transverse axis of the support arms and the pivot bearing of the backrest are coupled to one another by a gear mechanism installed in the housing-like frame of the backrest in such a way that the support arms are pivoted synchronously when the backrest is swivelled between the sitting position and reclining position and the respective inclination of the supporting arms relative to the seat level remains unchanged, - the transverse axes of the support arms are axially displaceably arranged within the frame of the backrest so that the distance between the support arms can be changed or adjusted in the transverse direction of the seat.