We evaluated the therapeutic efficacy of artemether-lumefantrine (AL) fixed-dose combination to treat uncomplicated Plasmodium falciparum malaria in Cruzeiro do Sul, Acre State, in the Amazon region of Brazil. Between December 2015 and May 2016, we enrolled 79 patients, 5-79 years old with fever or history of fever in the previous 48 hours and P. falciparum monoinfection confirmed by microscopy. Attempts were made to provide direct observation or phone reminders for all six doses of AL, and patients were followed-up for 28 days. AL was well tolerated, with no adverse events causing treatment interruption. All but one of the 74 patients who completed the 28-day follow-up had an adequate clinical and parasitologic response = 98.6% (95% CI: 93.2-100%). We could not amplify the one isolate of the case with recurrent infection to differentiate between recrudescence and reinfection. Five (6.3%) patients demonstrated persistent asexual parasitemia on Day 3, but none met definition for early treatment failure. We found no mutations in selected kelch13 gene domains, known to be associated with artemisinin resistance in P. falciparum isolates from Day 0. These results strongly support the continued use of AL as a first-line therapy for uncomplicated P. falciparum malaria in Acre. Routine monitoring of in vivo drug efficacy coupled with molecular surveillance of drug resistance markers remains critical.