Two patients developed severe zinc deficiency with acrodermatitis during parenteral hyperalimentation. The response of circulating T-lymphocytes to phytohemagglutinin was assessed both during the episode of clinical zinc deficiency and after intravenous zinc supplementation as the sole means of nutritional intervention. Maximum T-cell response to phytohemagglutinin, expressed as percent of simultaneous normal control response, was 2.1% and 27.9% in Patients 1 and 2 respectively. After 20 days of intravenous zinc supplementation (12 mg/d), repeat studies showed the T-cell response of Patient 1 to be 221% of the control, and that of Patient 2 to be 139% of control. In addition, Patient 1 was anergic during the period of zinc deficiency and normally reactive after zinc supplementation. These findings agree with extensive animal studies showing the detrimental effect of zinc deficiency on cellular immunity.