Much of modern portfolio theory rests on conclusions drawn from the original form of the Capital Asset Pricing Model. Fundamental to the conclusions of this model is the assumption of perfect competition among investors; i.e., all investors possess approximately the same small amount of wealth such that equilibrium price cannot be influenced significantly by the demand of any of the investors. Today's security market, however, is characterized by individuals and large institutional investors such as insurance companies and investment funds. Although institutions represent a very small fraction of all investors in the market, institutional investors in 1977 held 34.3 percent of all outstanding stock. By the very magnitude of the dollar transactions effected by these large investors, prices can and are affected dramatically. Because today's security market is composed of investors exhibiting extreme differences in wealth, the United States securities market probably is not perfectly competitive as assumed in portfolio theory. Consequently, investment theory must be extended to reflect vast wealth differences among investors. To achieve this end, modifications are made to the original Capital Asset Pricing Model. Equilibrium conditions are examined and conclusions are drawn as to how portfolio theory must be altered to include price affecting ability by a segment of the investors in the market.