The Project Baseline researchers are learning this firsthand. They say they have discovered and promptly alerted participants to potentially lethal conditions that might otherwise have gone unnoticed, like cancer and aortic aneurysms, so they can seek appropriate medical care. But some of the participants have also been frightened by fairly innocuous findings, like chest X-rays that reveal small, usually benign nodules in their lungs that they may look up on the internet and think are cancerous, said Dr. Charlene Wong, a Project Baseline investigator. “For most of our participants, it will not be cancer. But we’re still in the process of working with participants to find out if we can return that data in the right way so that we minimize the anxiety it can cause,” she said.

Dr. Ken Mahaffey, a Project Baseline investigator and cardiologist at Stanford, said that he and his colleagues have “a responsibility, socially, morally and ethically, to get systems in place so we can share the results with participants in ways that they can understand them and then help them engage with their own physicians and clinical providers.”

Despite the anxiety it can cause, many people welcome such data. Studies like Project Baseline are especially appealing to the so-called Quantified Self movement, the growing community of people who track their every biometric with smartphone apps, high-tech gadgets and direct-to-consumer health tests. Some 2,000 people have enrolled in Project Baseline so far, and thousands more have signed up in a registry of potential volunteers who may be called on as the project expands to additional medical centers.

While there have been plenty of longitudinal studies in the past, many of the largest and most important were not very diverse. The landmark Framingham heart study that began in 1948, for example, focused mostly on white adults. Dr. Svati Shah, an associate professor of medicine at Duke, said Project Baseline is recruiting many people who are black, Hispanic, Asian and other ethnicities so the study can shed light on differences in disease risk factors among people of different backgrounds.

That includes people like Rosa Gonzalez, 57, a nurse who lives in Concord, N.C. Ms. Gonzalez, who is Mexican-American, joined the study earlier this year and has encouraged at least a dozen Latino friends and acquaintances to join it as well.

“Other studies present data and talk about Latinos, but they don’t have Latinos in the study,” she said. “I’m trying to set an example so other Latinos see that it’s good to take part so that we can have data that shows how we’re the same or different.”

Dr. Gambhir said the idea for Project Baseline was hatched in 2013, when Google executives approached him and said they wanted to do a landmark study on human health. Dr. Gambhir proposed a study to find early markers of cancer in people who are otherwise healthy.