Hyaline articular cartilage from 102 thoracolumbar junction zygapophyseal joints was examined using light microscopy. One section from the upper, middle and lower third of each joint at the T10-11, T11-12, T12-L1 and L1-2 spinal levels was photographed using 35 mm color film. Hyaline articular cartilage cross-sectional area of the superior and inferior articular processes of each joint pair, at each level, was digitized using computer-aided planimetry. Differences in area were compared with joint orientation (symmetry or tropism) and geometry of each joint pair at each spinal level. Variation in hyaline cartilage cross-sectional area between joint pairs could be attributed to: a) geometric differences, b) the presence of a mortice joint, or c) osteoarthritic changes, with or without tropism. These findings were most frequently demonstrated at the transitional levels of T11-12 and T12-L1.