"In the first 24 hours of a stroke, that is the time where it is critical. We are still not sure how permanent this is or how severe this is. It does appear that she has lost the power of speech," her cardiologist Dr. Rodney Horton said.
Johnson's daughter, Luci Baines Johnson, said Johnson was alert and in stable condition Friday.
"She's seizing every moment of life and squeezing out the joy that is to be had," Luci Baines Johnson said.
An ambulance was called to Johnson's Austin home just after 5 p.m. Thursday when, after she woke from a nap, she had trouble swallowing some medication.
Her press assistant, Betty Tilson, said the 89-year-old former first lady also was having difficulty speaking and likely will be hospitalized for a few days.
"They have it as a mild stroke. She woke up this morning and was very alert," said Liz Carpenter, Johnson's former press secretary and a longtime friend.
Sandra Morgan, a spokeswoman for Seton Medical Center in Austin, said Johnson was in stable condition, "alert and in good spirits" at the hospital's intensive care unit.
Johnson visited with family members early Friday, Morgan said. But there was no indication when she would be released from the hospital.
"I think they're just being very cautious. She is 89," Tilson said.
"Seems to me she's doing pretty well," George Christian, who was President Johnson's press secretary and has been close friends with the former first lady for years.
Tilson said Secret Service agents administered oxygen to Johnson on Thursday after she started having trouble speaking and swallowing.
She said Johnson was scheduled to meet with a speech therapist Friday as doctors try to determine if she suffered permanent speech damage.
"The next 72 hours will tell the tale on that," Tilson said.
Granddaughter Nicole Nugent Covert accompanied Johnson in the ambulance.
"Her spirits were strong," Covert told the Austin American-Statesman. "When she heard my voice, she reacted to it. She looked at me and smiled."
Christian said that Johnson, who has been hospitalized in the past with heart problems, was remarkably upbeat.
"Seems to me she's doing pretty well," Christian said.
Johnson has visited with friends and family in the hospital, including Christian, her daughter and son-in-law Ian Turpin. Luci Johnson said her mother has been in and out of the hospital since January.
Turpin said Johnson has a pacemaker.
Johnson was admitted to the same hospital in 1999 after fainting at her home, and also underwent cataract surgery that year. In 1993, she suffered what was described as a minor stroke.
Her husband died in 1973. They had been married for almost 39 years.
While in the White House, from 1963 to 1969, Lady Bird Johnson served as honorary chairman of the national Head Start program and held a series of luncheons spotlighting women of achievement. But she was best known as the determined environmentalist who wanted roadside billboards and junkyards replaced with trees and wildflowers.
Although she has been in failing health in recent years, Johnson makes periodic public appearances at the LBJ Library and Museum and at civic events in Austin.
She also has remained active in her family business, the privately held LBJ Holding Co., but has handed the leadership duties of the broadcasting company to her daughter.