The death of a woman in Washington state in spring has been confirmed as the first US fatality from measles in 12 years, health authorities say.
Officials said the link was not spotted until a post-mortem examination because the woman lacked some of the common symptoms of measles, such as a rash.
Measles is highly contagious, but it has been rarely fatal in the US.
The death comes after an outbreak of measles at California's Disneyland in 2014 that infected over 100 people.
Although no one died from that outbreak, it sparked an intense debate across the US about mandatory vaccinations for schoolchildren, which some parents oppose.
Officials said the case in Washington state was not linked to that Disneyland incident.
The woman was on medication that had weakened her immune system, Washington State Department of Health spokesman Donn Moyer told reporters.
She was hospitalised for several health conditions in the spring at a facility in Clallam County, Mr Moyer said.
"She was there at the same time as a person who later developed a rash and was contagious for measles," he added.
Officials have not revealed her name or her age but did say that she was not elderly. An autopsy concluded the cause of death was pneumonia due to measles.
"This tragic situation illustrates the importance of immunising as many people as possible to provide a high level of community protection against measles," Mr Moyer said.
There have been 176 cases of measles in the US so far this year, with 117 of those cases linked to the Disney outbreak, according to the US Centers for Disease Control and Prevention.
The last US death from measles was in 2003.