A method and apparatus for automatically managing investment portfolios to substantially track a selected index and to automatically harvest tax losses is disclosed. Preferably, the system comprises an accounting system for maintaining tax lot information for individual accounts, an optimization system for rebalancing each account to substantially model the index and for harvesting tax losses, and a trading system for executing trades. Each investor owns the securities in his account, and therefore, harvested losses can be used to offset capital gains. The period between successive optimization procedures is selected to avoid application of the internal revenue service wash sale rules.