The 75-year-old monarch revealed he was battling cancer in early February and is continuing his treatment as he restarts his public-facing engagements.
The first fresh entry in his diary was a visit to a cancer treatment center in London, where he was expected to meet patients and staff.
Camilla, 76, has been president of cancer care and support charity, Maggie’s since 2008.
The King has largely remained out of the public eye during his treatment so far, apart from an outing on Easter Sunday when he delighted crowds with an impromptu walkabout after attending church with several family members.
Tuesday’s visit to University College Hospital Macmillan Cancer Centre was to reiterate the value of early diagnosis and focus attention on some of the innovative research taking place there.
The King’s outing came as he was announced as the new patron of Cancer Research UK. While at the center, he was due to meet with the organization’s chief clinician, Charlie Swanton, who has led a project called TRACERx, which is focused on lung cancer.