We report the occurrence of resistance to mefloquine 20mg/day in 51 children with falciparum malaria treated, at reference center of Manaus, Brazil, from October to December 1997. All children were evaluated at day 3, 5, 7, 14, 21, 28 and 35 of treatment. Clinical and parasitological cure criteria were adopted. The incidence of RIII mefloquine resistance was 5.9% (IC 95% 1.5-17.2). The cure/resistance proportion was 20:1 and cure/severity was 62:1. These findings suggest the importance of mefloquine resistance within this group of children.