Insider buying and selling. Stock purchases and dispositions by people “in the know” as an indicator of where prices are headed. Good info? Maybe – maybe not.

Far too many factors influence the validity of this indicator. Is an insider selling stock because s/he thinks it’s overvalued or is s/he disposing of shares because an option grant is about to expire? Maybe s/he’s diversifying. Maybe there’s a tax bill that has to be paid. A daughter’s wedding perhaps? A son’s college tuition?

The buy side is no better. Is stock being purchased because s/he thinks it’s undervalued or is there an ownership requirement imposed as a condition of employment?

The point is insider buying and selling like any other tool is nothing more than one piece in a complex puzzle. For those who believe it’s an important indicator we offer the following list of S&P 500 companies that have seen the largest dollar volume of insider buying over the past 90 days:

1. Legg Mason
The Baltimore-based money-management firm saw the highest level of dollar buying – $30.4 million. Its glory years were mid-decade when the stock topped out at $136/shr. Today it trades at about $28/shr.

2. EMC
The data storage giant saw insider buying of $12.4 million. The buyer?…the company itself! (Companies repurchasing their own stock are by definition ‘insiders.’) Each time you click your mouse the information has to be stored and chances are it’s on a machine manufactured by or running software from EMC.

3. Akamai
Entrant number three is a company providing products and services that speed and monitor internet traffic. The last 90 days have brought new insider money of $3.6 million.

Back in the late 90s and the days of the internet bubble Akamai traded as high as $327/shr before crashing and burning at less than $2/shr in 2002. Today you can pick up its shares at about $50.

4. Monsanto
Slotting just behind Akamai with $3.5 million of insider buying is the seed giant from St. Louis. The more mouths there are to feed the greater the demand for Monsanto’s products. Mangia!

5. Chicago Mercantile Exchange
CME Group (operator of the Chicago Mercantile Exchange) rounds out the top five with $1.5 million of insider buying. As financial products grow more complex and their uses (and users) grow in number CME Group stands to benefit as the largest exchange matching buyers and sellers.