Abstract: A multi-layer mattress comprises inflatable tubes each of which extends across a dimension of the mattress and form part of one of a number of arrays extending through each layer. All the tubes of any one array of which there are optimally three, are inflatable in common and are arranged in substantial vertical alignment with a corresponding array in another layer. Sequential inflation and deflation of each array causes ripples to travel along the mattress so that a patient lying on it has his weight-supporting areas continually changed. A preferred embodiment has two layers of tubes, each tube of which extends across a width of the mattress.