In an opinion issued Friday, Judge Jeffrey Lawrence said "a pre-embryo is a 'human being' ...whether or not it is implanted in its mother's womb."
He said the couple is as entitled to seek compensation as any parents whose child has been killed.
Alison Miller and Todd Parrish, who stored nine embryos in January 2000 at the Center for Human Reproduction in Chicago, filed the suit. Their doctor said one embryo looked particularly promising, but the Chicago couple was told six months later the embryos had been accidentally discarded.
In his ruling, Lawrence relied on the state's Wrongful Death Act, which allows lawsuits to be filed if unborn fetuses are killed in an accident or assault. "The state of gestation or development of a human being" does not preclude taking legal action, the act says.
Lawrence also cited an Illinois state law that says an "unborn child is a human being from the time of conception and is, therefore, a legal person."
"There is no doubt in the mind of the Illinois Legislature when life begins," Lawrence wrote.
Another judge had thrown out the couple's wrongful-death claims, but Lawrence reversed that decision, partly because that judge did not explain his decision at the time.
An attorney for the fertility clinic said an appeal would likely be filed.
The decision could curb reproductive research, said Colleen Connell, executive director of the American Civil Liberties Union in Chicago.
Connell expects the ruling will be overturned on appeal.
"It may be groundbreaking, but it's the wrong decision," Connell said. "No appellate court has ever declared a fertilized egg a human being in a wrongful-death suit."
Stem cells can potentially grow into any type of human tissue. Many scientists believe they could someday be used to repair spinal cord injuries and treat some diseases. Anti-abortion groups oppose such research because it involves destroying embryos, and the Bush administration has severely restricted federal stem cell funding.
Abortion opponents praised Lawrence's ruling. "Life begins at fertilization, not implantation," Pro-Life Action League director Joe Scheidler said.
While the ruling likely is too narrow to affect abortion law, it increases legal risks for fertility clinics, said John Mayoue, a family attorney in Atlanta and specialist on in-vitro law.
Mayoue said court rulings on the treatment of embryos have been contradictory.
"We are considering embryos to be property for certain purposes and life for others, and that's the incongruity," he said.