The Mexican Supreme Court has ruled that cases of disappearance during past campaigns to suppress dissent can be tried even if they are decades old.
Hundreds of activists disappeared during the so-called Dirty War
Previously, charges could not be brought in hundreds of cases from the rule of the Institutional Revolutionary Party, or PRI in the 1970s and 1980s.
Now charges can be brought even if the crime occurred before it became part of the penal code three years ago.
Charges can also be brought even if the victim has not been found.
Correspondents say the new ruling is a breakthrough for a special prosecutor investigating alleged abuses committed by PRI security forces, known as the "dirty war".
The PRI was defeated in elections in 2000 after 70 years in power.
The ruling is similar to a landmark decision on kidnapping last year.
Then the Supreme Court ruled unanimously that the statute of limitations did not apply to the 1975 kidnapping of the alleged rebel, Jesus Piedra, overturning an earlier decision by a lower court.
The move cleared the way for the possible arrest and prosecution of former officials implicated in the kidnappings of left-wing activists.