While individual health data will be kept confidential, Google may publish "anonymous, aggregated data" about Google Health users, said Roni Zeiger, product manager for Google Health. "If six months from now there are a lot of Google Health users, and we could publish a statement along the lines of '10 percent of diabetics last year also had a flu shot,' we would feel comfortable sharing that data," he said.

Google Health will be linked to some extent to other Google services, which could potentially also raise privacy or security concerns. For example, if a user creates a list of doctors' contact information, the group of contacts will automatically appear in their Gmail account, Zeiger said.

The service is available initially only in the U.S., he said. Google is talking to partners in Europe and Asia, but the service is more complex than others it offers, because of the sensitivity of the data and the number of partners involved, so it will take more time to be rolled out in other parts of the world, he said.

Users who sign up Monday won't have granular control over who sees specific parts of their health records, Zeiger said. Today it is "all or nothing" -- so when people authorize a provider to see their records they will be able to see all the information. The company plans to include granular control soon, but it wanted to launch the service now rather than wait, Zeiger said.

Google Health also includes some third-party services. They include a "virtual pill box" that will remind people via text message to their cell phone when to take their medicine. Users can also research illnesses. A search for "chicken pox" for example, provides results from sources like Google Scholar and Google Infrastructure.

Casey Kozlowski, Walgreens manager for health care automation, said Walgreens is allowing people to access their prescriptions history through Google Health. And if users allow it to, Walgreens will be able to view historical information such as allergic reactions people have had to drugs, she said.

"As a pharmacist, having an inaccurate or incomplete history of a person's prescriptions makes it really hard to do your job," she said.

Google announced a pilot of Google Health in February with the Cleveland Clinic, for between 1,500 and 10,000 participants. The original driving force for Google Health, Adam Bosworth, left the company last September. Mayer has been running the project until a permanent replacement is found.