Argentine President Cristina Fernandez de Kirchner will undergo surgery next week for cancer.
A spokesman said the cancer is in her thyroid gland but has not spread to other parts of her body.
Ms Fernandez, 58, recently began her second term as president after a landslide election victory.
Her husband - former President Nestor Kirchner - died last year after suffering a heart attack.
Ms Fernandez will step aside as president for 20 days until 24 January, with Vice-President Amado Boudou taking over.