Shares of the three companies below sell for more than 50 times their current-fiscal-year earnings forecasts. How expensive is that? There's no long-run historical average for forward price/earnings ratios because the process of polling forecasters to get a standardized earnings consensus is a fairly modern one, which is based on a measure of earnings that's more generous than the one used up until about two decades ago. However, the historic average for trailing P/Es is just under 15, and the average for forward-looking ratios should be lower than that. In other words, the companies below are more than three times as expensive as history's average stock.

Investors might pay such prices for stocks for a variety of reasons. They might think a company has suffered a temporary earnings setback that will be resolved in future years. The company might be barely profitable, resulting in earnings per share of just a few cents and a mathematically misleading P/E ratio. Of course, the company might simply be brimming with growth potential. In all three of those cases, investors believe near-term forecasts understate by far the company's long-term potential. For the list below, I started with the largest 1,500 U.S. companies by stock market value and ignored companies with recent setbacks or tiny earnings estimates, focusing instead on ones that are growing like gangbusters but which might nonetheless be priced for disappointment.

Salesforce.com

Forward P/E: 83

Salesforce.com sells companies access to web-based programs for planning and documenting sales and customer service calls and collaborating on projects. Its newly launched Chatter service is a social platform like Facebook and Twitter, but for work. The products allow companies to replace expensive installed software with simple pay-as-you-go access, and are proving popular. Sales are expected to increase 20% this year to $1.57 billion. Industry giants SAP and Oracle have sales of more than $13 billion and $26 billion, respectively, although they have much broader product lines.

Wynn Resorts

Forward P/E: 69

Wynn Resorts has a hit on its hands with Encore, its Macau casino that opened in April. Macau, a former Portuguese colony that like Hong Kong is now a special administrative region of China, is a renown high-roller haven. It's an hour from Hong Kong by ferry, or just under 20 minutes by helicopter. Wynn's surge in China is expected to more than make up for its slumping results back home in Las Vegas. Companywide sales are expected to jump 26% this year.

American Tower

Forward P/E: 53

American Tower builds and operates wireless communications towers. Industry trends are promising. The spread of phones that serve as mobile computers has left wireless operators with a voracious appetite for more bandwidth and broader networks. Tower operators are increasingly serving two or more wireless companies per tower, which brings in additional sales with a negligible rise in costs, resulting in sharply higher profits. American Tower is a large, financially stable operator that turns 40 cents of each sales dollar into operating profit a dime more than Apple. In addition to its lucrative U.S. business, the company has a presence in Brazil, Mexico and India, huge cellphone markets that for now significantly lag the U.S. in mobile broadband usage. Sales for American Tower are forecast to rise 10% this year.