The highest court in El Salvador has ordered President Salvador Sánchez Cerén to testify in the case of Archibald Gardner Dunn.
Dunn was serving as South African ambassador to San Salvador when he was seized by armed men outside the embassy on 28 November 1979.
Almost a year later, the left-wing rebel group Popular Liberation Forces said it had killed the diplomat.
President Sánchez Cerén was the group's second in command at the time.
In presidential elections held in March 2014, he became the first former rebel to lead El Salvador.
President Sánchez Cerén has not yet commented on the court's ruling ordering him to give evidence over Dunn's disappearance.
The lawsuit has been brought by Dunn's relatives, who are trying to find out what happened to the ambassador.
His family say that despite the payment of a $2m (£1.5m) ransom and the intervention of Archbishop Óscar Romero, the rebels did not reveal the whereabouts of Dunn.
On 8 October 1980, the rebel group instead circulated a statement saying they had killed the diplomat for "noncompliance with our demands". His body has never been found.
The kidnapping of Dunn, who was 60 at the time, happened at a time of political violence and upheaval in the Central American country following a military coup by reformist officers which ousted Gen Carlos Romero on 15 October 1979.
A 12-year-long civil war followed, pitting left-wing rebel coalition Farabundo Marti National Liberation Front (FMNL) against the US-backed Salvadorean army and right-wing death squads.
About 75,000 people were killed and 8,000 went missing before a UN-sponsored peace accord was signed in 1991.