Information is limited on the effect of zinc on immune responses in children with diarrhea due to enterotoxigenic Escherichia coli (ETEC), the most common bacterial pathogen in children. We studied the immunological effect of zinc treatment (20 mg/d) and supplementation (10 mg/d) in children with diarrhea due to ETEC. A total of 148 children aged 6-24 mo were followed up for 9 mo after a 10-d zinc treatment (ZT; n = 74) or a 10-d zinc treatment plus 3-mo supplementation (ZT+S; n = 74), as well as 50 children with ETEC-induced diarrhea that were not treated with zinc (UT). Fifty control children (HC) of the same age group from the same location were also studied. Serum zinc concentrations were higher in both the ZT (P < 0.001) and ZT+S groups (P < 0.001) than in the UT group but did not differ from the HC group. We found higher serum complement C3 immediately after zinc administration in both ZT (P < 0.001) and ZT+S (P < 0.001) groups than in the UT group. Phagocytic activity in children in both ZT (P < 0.01) and ZT+S (P < 0.01) groups was greater than in the UT group. However, oxidative burst capacity was lower in zinc-receiving groups (ZT, P < 0.001 and ZT+S, P < 0.001) than in the UT group. The naïve:memory T cell ratio in both ZT (P < 0.05) and ZT+S (P < 0.01) groups was higher than in the UT group from d 2 to 15. Increased responses, including complement C3, phagocytic activity, and changes in T cell phenotypes, suggest that zinc administration enhances innate immunity against ETEC infection in children.