Although oral rehydration therapy greatly reduces mortality from diarrhoeal diseases, it has little effect on stool frequency. However, there is mounting evidence that zinc is an effective adjunct to the treatment of diarrhoea, although few studies have examined its efficacy in Latin America. This study assessed the efficacy of zinc supplementation in children with acute diarrhoea in Brazil. The study was a double-blind, placebo-controlled, randomised, clinical trial in children <5 years of age attending emergency services in Sergipe, Brazil. Subjects received zinc or vitamin C as placebo. There was a marked reduction in the duration of the diarrhoea (1.1 vs 2.6 days) and of watery stools in the zinc-supplemented group. The efficacy of zinc was independent of the presence of viral enteropathogens in the stools. It is concluded that, similar to studies in India and Bangladesh, zinc could be an important adjunct for treating acute diarrhoea in Brazilian children.