Techniques and tools for encoding and decoding video images (e.g., interlaced frames) are described. For example, a video encoder or decoder processes 4:1:1 format macroblocks comprising four 8.times.8 luminance blocks and four 4.times.8 chrominance blocks. In another aspect, fields in field-coded macroblocks are coded independently of one another (e.g., by sending encoded blocks in field order). Other aspects include DC/AC prediction techniques and motion vector prediction techniques for interlaced frames.