If a European citizen stays in the U.S. for 90 days, and then leaves to travel for a week or more, can he or she then return to the U.S. for another 90 days?

If you are from one of these countries, you can enter US for 90 days without obtaining visa, to re-enter US after spending 90 days is usually not permitted.

Andorra Hungary New Zealand
Australia Iceland Norway
Austria Ireland Portugal
Belgium Italy San Marino
Brunei Japan Singapore
Czech Republic Latvia Slovakia
Denmark Liechtenstein Slovenia
Estonia Lithuania South Korea
Finland Luxembourg Spain
France Malta Sweden
Germany Monaco Switzerland
Greece the Netherlands United Kingdom


Visa Waiver Program (VWP) (http://travel.state.gov/visa/temp/without/without_1990.html)

You can try, but you will likely need to explain to the CBP Inspections Officer at the port of entry how you are truly a visitor. They can deny admission as you have already been in the US for 90 days and have only a short time out of the US.

If you have some detailed reason and some evidence with you about it, then you have a better chance. An example would be to attend a conference or some other long planned event. You will also need to show you have funds to pay for your stay in the US without having to work.