Zinc deficiency is an important factor that impairs cellular immunity and contributes to low T lymphocyte counts in head and neck cancers. Persistent T lymphopenia is clinically relevant in terms of tumor persistence and/or recurrence. The primary objective was to evaluate the impact of zinc sulfate supplementation on the absolute numbers of circulating T lymphocytes and T lymphocyte subpopulations. The secondary objectives were to evaluate overall survival, progression-free survival, and the adverse events of zinc sulfate. Seventy-two head and neck cancer patients were enrolled in a randomized, double-blind, placebo-controlled trial. Zinc sulfate 50 mg in 10 cc and an identically appearing placebo were self-administered 3 times daily at meal times. Blood samples were obtained for complete blood count, total T lymphocytes and T lymphocyte subpopulations before radiation therapy as baselines, at the fifth week during radiation therapy, and at the first month after completion of radiation therapy. The baseline characteristics of patients, tumors, and treatments and the baseline lymphocyte parameters were not significantly different between the 2 groups. Zinc sulfate supplementation during head and neck radiation therapy showed no increase in absolute numbers of circulating T lymphocytes, T lymphocyte subpopulations, or survival with acceptable side effects.