The United States Supreme Court is to rule in a case where a Virginia school district is fighting to stop a female born transgender student from using the boys' toilet.
The high school's board has appealed an earlier ruling favouring the student and the justices have now agreed to hear the appeal.
It is the first time the highest US court has agreed to rule on transgender rights.
In April, a lower court ruled that transgender students are protected under US laws barring sex-based discrimination.
The case involves a 17-year-old transgender student named Gavin Grimm, who identifies as male and sued in 2015 to win the right to use the Gloucester County school's boys' toilet.
"I never thought that my restroom use would ever turn into any kind of national debate. The only thing I ever asked for was the right to be treated like everyone else," he said in a statement.
Gavin's lawyer, Josh Block, said the court's decision to take the case means he will not be able to use the boys' toilet before graduating from high school next year.
"These sorts of discriminatory policies stigmatise and isolate transgender students like Gavin just because of who they are," Mr Block said.
The school board's chairman, Troy Anderson, welcomed the court's decision to hear the case, saying the board's policy "carefully balances the interests of all students and parents".
Conservative groups have backed the school board, saying the fight is about student privacy rights.
"In light of the right to bodily privacy, federal law should not be twisted to require that a male be given access to the girls' facilities, or a female to the boys' facilities," said Gary McCaleb, a lawyer with the Alliance Defending Freedom, a conservative Christian legal group.
US President Barack Obama's administration concluded that transgender people are protected by the ban on gender discrimination, but a legal fight has ensued over whether judges are required to defer to the government's view.
The justices could side-step a major ruling by focusing their decision on whether the administration followed the correct procedure when it gave public notice about that legal conclusion.
Supreme Court still shorthanded
The Supreme Court remains one justice short following the death in February of Antonin Scalia, which left it with four conservatives and four liberals.
That raises the possibility of a 4-4 ruling that would leave in place an earlier decision favoring Gavin Grimm by a Court of Appeal. However, a 4-4 ruling would not set a nationwide legal precedent.
The court has until now steered clear of taking potentially divisive cases while it remains shorthanded.
Although it has not directly ruled on transgender rights before, the Supreme Court did rule in 1994 in favour of a transgender prison inmate who was held with male prisoners and said she was beaten and raped by another inmate.
Whether to allow transgender people to use public toilets that correspond to their gender identity rather than their birth gender has become the latest flashpoint in the long US battle over lesbian, gay, bisexual and transgender rights.
In a major gay rights ruling in 2015, the Supreme Court legalised same-sex marriage nationwide.
The transgender fight heated up in March after North Carolina passed a Republican-backed law requiring people to use toilets that correspond to their gender at birth in government buildings and public schools.
The North Carolina law, being challenged in court, also blocked local measures protecting lesbian, gay, bisexual and transgender people from discrimination.
The Obama administration in May issued nationwide guidance telling public schools that transgender students should be allowed to use whichever toilet they chose.
That guidance infuriated many conservatives and prompted a Republican-led legal effort to fight it.
Twenty-three states sued to block the guidance.
In August, a US district court judge issued a nationwide injunction sought by Texas and other states preventing the administration from enforcing the guidance.
Used boys' toilet without incident
In the Virginia case, the Supreme Court in July voted 5-3 to temporarily block the appeals court decision from going into effect while the litigation continued, a move that prevented Gavin Grimm from using the boys' toilet when the new school year began in September.
Gavin Grim started attending school as a boy in September 2014. With the school's permission, he used the boys' toilet for about seven weeks without incident.
But after complaints from parents, the county school board adopted a new policy in December that year which required students to use the toilet that corresponded with their gender at birth.
Since then, he has had to use a separate toilet.
As a result of hormone therapy, Gavin Grimm has facial hair and a deep voice.
He has also had chest reconstruction surgery but not sex reassignment surgery.