Pyramid data structures have found an important role in progressive image transmission. In these data structures, the image is hierarchically represented, with each level corresponding to a reduced-resolution approximation. To achieve progressive image transmission, the pyramid is transmitted starting from the top level. However, in the usual pyramid data structures, extra significant bits may be required to accurately record the node values, the number of data to be transmitted may be expanded, and the node values may be highly correlated. In this paper, we introduce a reduced-difference pyramid data structure in which the number of nodes, corresponding to a set of decorrelated difference values, is exactly equal to the number of pixels. Experimental results demonstrate that the reduced-difference pyramid results in lossless progressive image transmission with some degree of compression. By use of an appropriate interpolation method, reasonable quality approximations are achieved at a bit rate less than 0.1 bit/pixel and excellent quality approximations at a bit rate of about 1.3 bits/pixel.