This study aimed to determine the incidence rates of refeeding phenomena (defined as a decline in p-phosphate) and refeeding syndrome (RFS; defined as development of clinical symptoms in addition to a decline in p-phosphate) in head and neck cancer patients, and to identify risk factors. Fifty-four head and neck cancer patients referred for surgery were included. Forty-six potential risk factors were registered at the baseline, and p-phosphate was measured at Days 2, 4, and 7. Eleven patients (20%) developed RFS, and twenty-eight (52%) developed refeeding phenomena. At baseline, these patients presented a higher prevalence of head and neck pain, eating difficulties, higher p-phosphate levels, lower p-transferrin levels, and, in men, lower b-hemoglobin levels. Patients who developed symptoms had a decline in p-phosphate ≥0.22 mmol/l. At baseline, these patients had higher p-phosphate levels, higher alcohol consumption, and lower p-transferrin and p-sodium levels, as well as a higher prevalence of eating difficulties, low handgrip strength (HGS), and a history of radiation therapy. The risk factors most strongly associated with the development of refeeding phenomena and RFS were pain, eating difficulties, low HGS, high alcohol intake, and previous radiation therapy.