Growth rates of rat tendon fibroblasts cultured in a three-dimensional carbon fiber matrix were compared with those of cells cultured on standard flat culture plates. The carbon fiber has been used as a tissue scaffold for tendon and ligament repair in animal and clinical studies. While cell growth on the culture plates appears to follow a growth curve containing a lag phase, a log phase, and plateau phase of growth, cell growth in the fiber matrix was characterized by a suppressed log phase of growth. SEM and cytotoxicity studies indicated that this effect was not caused by growth-inhibiting or cytotoxic substances from the carbon fiber. While we cannot rule out the possibility that cell growth was influenced by the surface chemistry of the carbon substrate, evidence from this and other studies suggests that the observed effect was caused by a lack of readily available surface area for cell attachment and growth on the small fibers. Because cell colonies growing on individual fibers are limited (at least in theory) to growing in two directions only, they enjoy limited opportunities for cell migration and growth--in contrast with cell colonies on flat culture plates. These results suggest fundamental differences in the mechanisms controlling cell growth on planar vs. three-dimensional fiber substrates.