The relationship between cell shape, proliferation, and phenotypic expression was studied in human chondrocytes. Shape was controlled independent of serum concentration, anchorage, and cell density by alteration of substratum adhesiveness with poly(2-hydroxyethyl methacrylate) (poly[HEMA]). Cells that were held rounded displayed features of the chondrocyte phenotype; i.e., they were round, proliferated slowly, incorporated low levels of [3H]thymidine into DNA, and incorporated large amounts of 35SO4 into glycosaminoglycans. In contrast, cells that were held flat were fibroblast-like: they exhibited flattened morphology, more rapid growth, greater incorporation of [3H]thymidine, and less incorporation of 35SO4. These studies suggest that cell shape may play an important role in phenotypic expression in chondrocytes.