Zinc deficiency (serum zinc level 65 micrograms/dl) and cellular immune deficiency (confirmed by lymphoblastic transformation assay) were found in a 15-year-old boy who had had recurrent aphthous ulceration for 6 years. Despite the previous therapy (orally administered steroid, isoprinosine, interferon, lysozyme, and local treatment), the ulcers recurred monthly. After 3 months of zinc therapy (50 mg zinc sulfate orally three times daily) the aphthae disappeared and did not reappear for 1 year. The lymphoblastic transformation activity and serum zinc levels were also normalized with this treatment.