This study compares treatment of uncomplicated malaria in children in Guinea-Bissau as recommended by the national malaria programme (chloroquine in a total dose of 25 mg/kg), either with a total dose of 50 mg/kg chloroquine or with a total dose of 15 or 30 mg of amodiaquine. As both annual in vitro studies (from 1992 to 2004 except 1998, 1999) and several in-vivo studies from Guinea-Bissau indicate a fairly stable chloroquine resistance prevalence, another aim of this study is to evaluate the genetic basis of chloroquine resistance in Guinea-Bissau by analyzing specific single nucleotide polymorphisms in pfcrt and pfmdr1 in blood samples from this in vivo trial.