The 89-year-old Lee was admitted to Singapore General Hospital on Friday, The Wall Street Journal reported. The office of his son, Prime Minster Lee Hsien Loong, released a statement saying the elder Lee suffered "a suspected transient ischaemic attack associated with a prolonged episode of atrial fibrillation" that briefly stopped the flow of blood to his brain.
Lee was expected to remain in the hospital for several days.
Lee was prime minister from 1959 to 1990 and remained in the Cabinet until 2011. He is still a legislator.