The strike — which jumped from three participants in late May to 89 on Thursday — was the biggest of the year at the U.S. prison in Cuba, where about 460 men are being held on suspicion of links to Al Qaeda or the Taliban.
The U.S. military has said the detainees were trying to pressure the United States to release them. A human rights attorney has described the strike as a desperate appeal for justice.
Four hunger strikers were being force-fed, Navy Cmdr. Robert Durand said in a statement.