In this paper, we investigate the statistical properties of interlaced and progressive image sequences in order to determine what is the most efficient format from a coding point of view. Intra- and inter-coding modes are both considered. The main result is that, for both intra- and inter-images, the coding of progressive images outperforms the coding of interlaced ones for low bit rates and large motion vectors. Particular attention is paid to inter-coding with subpel accuracy, where the effects of nonideal interpolation filters and finite precision of the motion vector are taken into account.