After more than three weeks in the hospital, the 88-year-old Pope is responding well to treatment and has shown a "gradual, slight improvement" in recent days.
Francis on Sunday followed the opening day of a week-long spiritual retreat in the Vatican for Holy See officials by video conference.
He could see the officials, but they couldn't see him in the Gemelli hospital, the Vatican press office said.
Francis also participated in Mass at the hospital's private chapel.
For the fourth consecutive Sunday, Francis didn't appear for his weekly noon blessing, but the Vatican distributed the text he would have delivered if he were well enough.
In it, the Pope thanked all those who were caring for him and others who are sick and experiencing a "night of pain".
"Brothers and sisters, during my prolonged hospitalisation here, I too experience the thoughtfulness of service and the tenderness of care, in particular from the doctors and health care workers, whom I thank from the bottom of my heart," read the message from Gemelli hospital.
"And while I am here, I think of the many people who in various ways are close to the sick, and who are for them a sign of the Lord's presence.
"We need this, the 'miracle of tenderness' which accompanies those who are in adversity, bringing a little light into the night of pain," he wrote.
The pontiff, who has chronic lung disease and had part of one lung removed as a young man, has remained in stable condition, with no fever and good oxygen levels in his blood for several days, doctors reported.
The doctors said that such stability "as a consequence testifies to a good response to therapy".
It was the first time the doctors had reported that Francis was responding positively to the treatment for the complex lung infection that was diagnosed after he was hospitalised on February 14.
But they kept his prognosis as "guarded", meaning that he's not out of danger.
In his absence, the Vatican's day-to-day operations continued alongside celebrations of its Holy Year, the once every quarter-century Jubilee that brings millions of pilgrims to Rome.
On Sunday, Canadian Cardinal Michael Czerny celebrated the Holy Year Mass for volunteers.
During the Mass in St Peter's Square, the giant banner bearing Francis' papal coat of arms fluttered from the loggia of the basilica above.
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