We have developed an in vitro endothelial cell (EC)-smooth muscle cell (SMC) coculture platform that can mimic either the healthy or diseased state of blood vessels. Transforming growth factor-beta1 (TGF-beta1) and heparin were introduced to the SMC cultures to upregulate the SMC differentiation markers, alpha-smooth muscle actin (alpha-SMA) and calponin (homotypic model). Interestingly, seeding of near-confluent concentrations of ECs on the SMCs (heterotypic model) induced higher levels of alpha-SMA and calponin expression in the SMC cultures than did the addition of heparin and TGF-beta1 alone. The expression levels increased further on pretreating the SMCs with TGF-beta1 and heparin before adding a near-confluent monolayer of ECs. In contrast, seeding of sparse concentrations of ECs forced the SMCs into a more hyperplastic state as determined by alpha-SMA and calponin expression. This study highlights the importance of both soluble factors and EC seeding densities when considering culture conditions; in vivo SMCs are in close proximity with and interact with a monolayer of ECs. Our study suggests that this architecture is important for healthy vascular tissue function. In addition, it shows that disruption of this architecture can be used to mimic diseased states. As the EC-SMC coculture model can mimic either a diseased or a healthy blood vessel it may be useful as a test bed for evaluating cardiovascular therapeutics.