Supreme Court Justice Ruth Bader Ginsburg, 85, has been hospitalised after she fractured three ribs in a fall in her Washington office.
The court's oldest justice fell Wednesday evening but went to George Washington University Hospital after experiencing discomfort overnight, court spokeswoman Kathy Arberg said.
She was admitted to the hospital for treatment and observation after tests showed she fractured three ribs.
In her absence, the court went ahead Thursday with a courtroom ceremony welcoming new Justice Brett Kavanaugh, who joined the court last month.
Donald Trump, the US president, and new acting Attorney General Matthew Whitaker both attended the ceremony.
Mr Kavanaugh's appointment restores the nine-member court's conservative majority. The court has the final say on interpreting the constitution when it comes to issues such as abortion, gun control and voting rules.
Appointed by President Bill Clinton in 1993, Mrs 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.
Mrs Ginsburg has had a series of health problems. She broke two ribs in a fall in 2012. She has had cancer twice and had a stent implanted to open a blocked artery in 2014. She also was hospitalised after a bad reaction to medicine in 2009.
But she has never missed Supreme Court arguments. The court, which resumed last month, will not hear arguments again until November 26.
Rib fractures are common among older adults, particularly after falls, but the extent of Mrs Ginsburg's injury is not yet clear.
Broken ribs typically heal on their own in six weeks to a month, and patients are advised to limit strenuous activity.
But they can be very painful and controlling pain is key. A chief complication is pneumonia, when patients don't breathe deeply enough or cough enough because of the rib pain.