King Gyanendra named Girija Prasad Koirala as the Himalayan kingdom's new prime minister on Thursday.
Koirala was the choice for prime minister by the alliance of political parties behind weeks of bloody protests that forced Gyanendra to restore democracy.
But illness could keep Koirala from Friday's first session of the reinstated parliament.
Krishna Sitaula, Koirala's spokesman, offered few details of the PM-designate's ailment, saying only that it was a lung problem.
Koirala's illness kept him away from a victory rally in Katmandu, marring a day that began with the Maoist rebels announcing a three-month cease-fire.
in a victory rally in Kathmandu
The truce lifted a burden on the new government poised to take control after protests in which 15 people were killed by soldiers and police.
However, the Maoists were clear that the cease-fire is only for three months and "with the intention to encourage the political parties to announce an unconditional special assembly" to rewrite Nepal's constitution, which enshrines the monarchy as the key arbiter of power in the country.
They also appear to believe the new government will give them a chance to play a legitimate public role.