Nepal's Prime Minister Sushil Koirala has been diagnosed with lung cancer.
He is currently receiving treatment at a hospital in New York, where doctors have recommended he undergo a course of radiotherapy.
Mr Koirala, 75, smoked heavily until he was diagnosed with cancer of the tongue eight years ago, reports say.
He was elected prime minister in February and is tasked with drafting a new constitution in the Himalayan nation by the end of the year.
"The spot seen in one of his lungs has turned out to be first stage of lung cancer," the prime minister's personal physician, Dr Karbir Nath Yogi, who is accompanying him in New York, told BBC Nepali.
Dr Yogi said surgery was not an option because of the prime minister's chronic bronchitis.
"Radiotherapy will be equally effective. We expect he will have to remain in New York for 10-12 days, after which he will return home and continue the treatment."
Mr Koirala leads Nepal's largest political party, the Nepali Congress. No party won a majority in Constituent Assembly elections last year.
Attempts to agree a new constitution have been dogged by years of wrangling since the country became a republic in 2008, following a peace deal with Maoist rebels in 2006.