An opinion piece in the magazine said aid agencies were often highly competitive, were "polluted by internal power politics" and were "obsessed with raising money through their own appeal efforts".
But Brendan Gormley, chief executive of the DEC, said the aid agencies had coordinated their response effectively.
"Rather than jostling for position, 13 major UK aid agencies have come together under our banner to launch the DEC Haiti earthquake appeal to ensure a coherent and united response in our UK fundraising and publicity," he said in a statement.
"It is clear that the scale of the disaster in Haiti is far too large for any individual agency and the DEC agencies always work closely together on the ground and coordinate effectively with governments, the UN and affected communities."
Gormley said The Lancet failed to take into account the difficulty of the work in Haiti and the huge effort put in by staff.
He said aid agencies did have to undergo scrutiny, contrary to suggestions in the magazine.
"Just last month, the DEC published an independent report into the reconstruction work in Aceh, Indonesia, after the 2004 tsunami," he said. "The report by respected consultants at Arup confirmed the great work that had been done."