Halamka hopes that as more health-care providers sign up as partners, services like Google Health can allow patients to build a secure, seamless lifetime record of their medical information.

In its first week of operation, Halamka says, Google Health attracted about 150 of the several hundred thousand patients Beth Israel Deaconess sees each year. That's not a lot. But if the service proves as popular as PatientSite, which has more than 40,000 users, it could give a big boost to Google's efforts.

One of the biggest unknowns--and a topic of great debate--is whether people will trust their medical information to Google. Because it is neither a health-care provider nor an insurer, Google Health is not covered by the Health Information Privacy and Accountability Act (HIPAA). "If protections are not in place, there is always the risk that data will be used not in the best interest of the patient, but rather in the interest of the bottom line," says Isaac Kohane, chair of informatics at Children's Hospital Boston.

As Google Health grows, it will attract not just hospitals and pharmacies, but also other kinds of third-party partners seeking access to their members--raising the specter of new routes for direct-to-consumer marketing of drugs, unscrupulous research, and data mining.

Other hospitals will be watching carefully before they choose to partner with services like Google Health, says Kenneth Mandl, who runs Indivo, the PHR platform used at Children's Hospital Boston. "We need to see the proper framework for regulation, certification, and privacy emerge that covers all platforms," he says. "At that point, I think that ultimately it should be a patient's choice about what platform they use."

Halamka says that the company is taking critics' concerns very seriously. "Google has been really pretty compulsive about privacy," he says. "If it violates patient trust, you might as well just shut the site down."