Methods: An in vivo drug efficacy study will be conducted in Cruzeiro do Sul, Acre State, Brazil. A total of 117 study participants ≥5 years of age with parasitologically confirmed P. vivax monoinfections will be treated under supervision with chloroquine (CQ) for three days at a daily dose of approximately 25 mg/Kg in accordance with the Brazilian National Malaria Control guidelines. For patients with normal glucose 6 phosphate dehydrogenase activity levels, investigators will add primaquine at dose of 0.5mg/Kg per day for 7 days. Clinical and parasitologic parameters will be monitored over a 28-day follow-up period to evaluate drug efficacy and for a total period of 168 days (6 months) to evaluate chances of recrudescence, relapse, or reinfection. Blood samples will be taken to measure the CQ levels in blood on Day 7 and day of failure, if occurring in the initial 28 days of follow up. In addition, a blood sample will be collected on filter paper on first day and on day of suspected failure to help differentiate parasite genotypes using techniques based on polymerase chain reaction. Results from this drug efficacy study will be used to assist the Brazilian Ministry of Health in assessing their national malaria treatment policy for P. vivax malaria.