Phase II Trial of Combination Anti-PD-1 and Aldesleukin for Metastatic Melanoma and Renal Cell Carcinoma

Background: Aldesleukin is used to treat metastatic or advanced melanoma and renal cell carcinoma. Pembrolizumab is used to treat many cancers including melanoma. Researchers want to see if these drugs can be used together to produce better results in people with these types of cancer. Objective: To learn if the combination of pembrolizumab and aldesleukin can be used to treat metastatic or advanced melanoma and renal cell cancer. Eligibility: Adults aged 18 years or older who have metastatic or advanced melanoma or renal cell carcinoma. Design: Participants will be screened with: * Medical history * Physical exam * Electrocardiogram * Blood and urine tests * Ability to perform tasks of daily living * Imaging scans (CT, MRI, PET, and/or X-rays). They may get a contrast agent to enhance the images. * Photographs, if needed Some of these tests will be repeated during the study. Participants will receive the study drugs by IV (a plastic tube that is put into a vein) for 4 days. A second cycle of treatment will be given 21 days later. They will stay in the hospital for each of the cycles in the first course of treatment. After 2 months, their cancer will be evaluated. They may receive a second course of pembrolizumab alone on Days 1 and 21. They will not have to stay in the hospital for this course. About 30 days after treatment ends, participants will have a safety follow-up visit. Then they will have visits every 3 months for up to 1 year, and then every 6 months for up to 4 years. Follow-up can also be done by phone, email, and mail. If their cancer gets worse, they will stop having visits. Participation will last for 5 years.