STUDY OBJECTIVE: To explore the rapid rise of the extremely old population, showing the magnitude of the increase and identifying demographic mechanisms underlying this increase. DESIGN: Demographic analysis using census data, yearly population estimates, and mortality statistics. SETTING: Switzerland 1860-2001. MAIN RESULTS: Indicators suggest a strong increase in the number of nonagenarians and centenarians in Switzerland as compared with other countries. The increase is mostly attributable to the decline in mortality after age 80. This decline started in the 1950s. CONCLUSION: Nonagenarians and centenarians constitute a new population, which became sizeable after 1950 in Switzerland. There is a need to monitor this population with appropriate demographic and epidemiological indicators.