The US Supreme Court has upheld a law allowing doctors in the state of Oregon to help terminally ill patients die, in a defeat for the Bush administration.
Mr Roberts was overruled in his first case as chief justice
Justices voted 6-3 to back the law, under which doctors are thought to have assisted with at least 208 suicides.
The ruling could free other states to pass laws like Oregon's, which is the only one of its kind in the US.
New Chief Justice John Roberts was in the minority in the court's first major case on ethics since he joined it.
Experts had debated which way the chief justice would vote.
Many had expected him to oppose the law because he is a Roman Catholic, but others thought he might back it as an advocate of a state's right to govern its own affairs.
Patients must be in final six months of terminal illness
Patients must make two oral requests and one written request to die, separated by a two-week period
Patients must be mentally competent to make decision
Two doctors must confirm diagnosis
Lethal prescription of drugs prescribed by doctor and administered by patients themselves
Justice Anthony Kennedy wrote the majority decision, which is being seen as a rebuke for President George W Bush's former Attorney General, John Ashcroft, who brought the case to court.
Mr Ashcroft had sought to use laws against drug dealing to punish doctors who prescribe fatal doses of medication under the Oregon law.
Justice Kennedy wrote that the "authority claimed by the attorney general is both beyond his expertise and incongruous with the statutory purposes and design".
Justice Antonin Scalia, who wrote the minority, dissenting view, said: "If the term 'legitimate medical purpose' has any meaning, it surely excludes the prescription of drugs to produce death."
Oregon passed its law in a 1997 state referendum.
Since then, most of those who have chosen to die under the law have been cancer patients.
Under the Death with Dignity Act, a patient must have less than six months to live, must be deemed by two doctors as mentally fit to make the decision, and must present one written and two oral demands over a certain period.
He can then obtain lethal medication from his doctor but must administer it himself.
Touched by cancer
Many of the nine justices of the Supreme Court have personal experience with cancer.
Three have had the disease and a fourth has a wife who counsels dying young cancer patients.
Mr Roberts' predecessor, William Rehnquist - who was chief justice for 18 years - died of cancer in September.
Mr Kennedy was joined in the majority by John Paul Stevens, Stephen Breyer, David Souter, Ruth Bader Ginsberg and Sandra Day O'Connor, who is retiring.
Mr Roberts, Mr Scalia, and Clarence Thomas voted against the Oregon law.
The US Senate is currently considering whether to approve Judge Samuel Alito to replace Ms O'Connor.