The Supreme Court has said 85-year-old Justice Ruth Bader Ginsburg fractured three ribs in a fall in her office at the court and is in hospital.
The court has said the justice went to George Washington University Hospital in Washington early on Thursday after experiencing discomfort overnight.
The court says the fall occurred on Wednesday evening.
Ms Ginsburg was admitted to the hospital for treatment and observation after tests showed she fractured three ribs.
Ms Ginsburg broke two ribs in a fall in 2012.
She has had two prior bouts with cancer and had a stent implanted to open a blocked artery in 2014.
The Supreme Court is a powerful body which has the final word on issues like abortion and same-sex marriage.
In Ms Ginsburg’s absence, the court was going ahead on Thursday with a courtroom ceremony welcoming new Justice Brett Kavanaugh, who joined the court last month after a fiercely-contested nomination process.
President Donald Trump and new acting Attorney General Matthew Whitaker were expected to attend.
Appointed by then-president Bill Clinton in 1993, Ms Ginsburg rebuffed suggestions from some liberals that she should step down in the first two years of President Barack Obama’s second term, when Democrats also controlled the Senate and would have been likely to confirm her successor.
She already has hired clerks for the term that extends into 2020, indicating she has no plans to retire.