Chloroquine-resistant P. vivax was first reported from Papua New Guinea in 1989 in two Australian soldiers (Rieckmann, Davis et al. 1989). In 1995, a study in Irian Jaya, Indonesia showed resistance in at least 44% of the P. vivax patients treated with chloroquine (CQ) (Baird, Basri et al. 1995). Several investigators have reported cases of CQ-resistant P. vivax in South America. In 1996, in Guyana, Phillips et al. reported three patients in whom 25 mg/kg of CQ failed to eliminate parasitemia despite adequate therapeutic blood levels of CQ (Phillips, Keystone et al. 1996). Three years later, in the Brazilian Amazon region, Alecrim et al. reported a 12-year old girl with P. vivax malaria who continued to have parasitemia after receiving a supervised course of 25mg/kg of CQ (Alecrim, Alecrim et al. 1999). More recently, Soto et al. reported three cases of CQ-resistant P. vivax in Colombia (Soto, Toledo et al. 2001). In these last two studies, CQ blood levels were not measured and it is not possible to confirm that adequate therapeutic levels were achieved.