HealthWise Wales survey on nation's future health needs
People in Wales are being asked to take part in a confidential online survey to help plan health needs.
The HealthWise Wales project, described as the first of its kind in Europe, hopes to collect information from 260,000 people.
The aim is to build an in-depth picture of the health of the nation, using the data to plan NHS services in future.
Anyone over 16 can take part and the Welsh government said the data would only be used for research.
The project is led by Cardiff and Swansea universities, and some participants will be asked if researchers can access their information and invite them to take part in further studies.
Dr Ruth Hussey, Wales' chief medical officer, said it was the largest research project of its kind ever launched in Wales.
"Young or old, fit or unwell you will be contacted every six months to get an ongoing understanding of your health," she said.
"This is about people sharing their knowledge to improve health and care in Wales."
A pilot was launched in May 2015 and 570 people have signed up to date.
It aims to recruit hundreds of thousands more over the next five years by encouraging people to register online, as well as a mixture of roadshow events, TV adverts and social media campaigns.
Prof Shantini Paranjothy, from Cardiff University's School of Medicine, said the project built on "a strong tradition of population health research in Wales".
"This large-scale project will use modern technology to engage people in research and provide opportunities to contribute to the design and conduct of research studies," she said.