The procedure Sen. John McCain underwent last week for a blood clot near his eye revealed a primary brain tumor, according to his office.
The tumor is known as a glioblastoma and was associated with the clot, a release from his office said.
The American Brain Tumor Association says glioblastoma is a highly malignant cancerous tumor.
Treatment options could include a combination of chemotherapy and radiation.
The office's statement said doctors called his underlying health excellent aside from the tumor.
It's not clear when McCain will return to the senate.
McCain's office also released the following statement:
"Senator McCain appreciates the outpouring of support he has received over the last few days. He is in good spirits as he continues to recover at home with his family in Arizona. He is grateful to the doctors and staff at Mayo Clinic for their outstanding care, and is confident that any future treatment will be effective. Further consultations with Senator McCain's Mayo Clinic care team will indicate when he will return to the United States Senate."