A roller unit (12) comprises a connector assembly secured to a structure or to a door. The connector assembly comprises a spindle (32). The roller assembly has a wheel (60) having a rolling-element bearing (61) mounted to the spindle (32), and an annular wheel body (62) mounted to the rolling-element bearing (61) for rotation about the spindle (32). The annular wheel body (62) has a pair of flanges (64,68) separated by a groove (63) on a circumferential surface of the annular wheel body. The circumferential surface defines a shape corresponding to the shape of an edge portion of the door or structure engaged in the annular wheel body. The connector assembly is connected to one of the door and the structure, while the wheel receives an edge portion of the other of the door and the structure, whereby the roller unit (12) holds the door and structure face to face while allowing sliding movement of the door with respect to the structure by rotation of the wheel (60) about the spindle (32). A sliding door system is also described.