"I regret not to have been able to travel to our sister nation Bolivia -- a bout of the flu has prevented me," Maduro tweeted.
Maduro had been expected in Bolivia for regional talks with local counterpart Evo Morales and Ecuadoran President Rafael Correa.
"We will see each other soon, beloved fellow presidents," he wrote, adding that the leftist stalwarts with a shared hatred of the United States would "forever be united in solidarity."
La Paz's Foreign Minister David Choquehuanca told reporters in Cochabamba -- the summit's host city 400 kilometers east of the Bolivian capital -- that he had been informed "at the last minute" that Maduro would not be coming, and regretted his absence.
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Correa, who arrived at dawn in Cochabamba, was participating in a rally in the town of Ivirgarzama, a major coca producer and political bastion of Morales.
"We will strengthen our relations and the brotherhood between the two countries," the Bolivian foreign minister said.
The announcement follows Maduro's last-minute decision to forego the UN General Assembly meeting in New York last month, saying he was fearful of a US plot against his life.
Caracas and Washington have recently engaged in tit-for-tat expulsions of diplomats, amid worsening relations.