AbstractRecently, reports of delayed hemolytic anemia after treatment with artemisinin and its derivatives have emerged. Here we report two cases of delayed hemolytic anemia in a patient with severe falciparum malaria after treatment with oral artemether-lumefantrine (AL). The first patient, a 20-year-old Japanese male student, was diagnosed with falciparum malaria and was administered AL. As having a high parasitemia rate (20.6%) was the only severe malaria criterion met in this case and his general condition was stable, we continued with AL treatment. Despite disappearance of malarial parasites after 4 days of AL administration, a persistent fever remained. On days 13 and 16, a diagnosis of hemolytic anemia was made (lactate dehydrogenase [LDH]: 1,466 U/L, hemoglobin [Hb]: 7.2 g/dL). A blood smear at that time revealed no parasites. He recovered naturally from delayed hemolysis. The second patient, a 27-year-old Japanese female student, was diagnosed with falciparum malaria (parasitemia: 4.5%) and treated initially with oral quinine hydrochloride and doxycycline. The following day, parasitemia increased to 7.9% and oral AL was initiated. She was discharged on day 4 after achieving parasite clearance and afebrility. However, on day 5, fever (body temperature > 38°C) recurred, and on day 11, a diagnosis of hemolytic anemia was made (LDH: 712 U/L, Hb: 8.8 g/dL). A follow-up confirmed that her condition improved gradually. AL treatment of severe malaria can cause delayed hemolytic anemia. Patients should be followed up for up to 4 weeks to detect signs of hemolysis and provide appropriate symptomatic treatment.