Abstract: An upholstered seat assembly and also a cushion and a one-piece seat and back shell for the assembly and molded of thin-walled plastic. The shell has a seat and an upwardly extending back, a continuous edge flange formed around said seat and back and formed integrally therewith. The cushion is secured to said flange, and the back and seat have side walls for the reception and support of the cushion. The seat has a plurality of generally parallel curvilinear slots which demarcate a plurality of generally parallel curvilinear spring segments extending transversely across the seat. The seat is sloped to have a positive bow for the segments to permit them to deflect downwardly in response to a load created by the occupant of the upholstered seat. The seat also has an underside and a generally rectangular and downwardly extending frame formed on the seat underside and which provides a mounting surface for the upholstered seat.