2008, and Dec 28, 2010, and at Salavan provincial hospital in southern Laos between Sept 5, 2008, and Dec 26, 2010. The hospitals are roughly 770 km apart. Luang Namtha has 60 beds and serves a population of about 145 000 individuals from the highlands of the China–Burma border. Salavan has 70 beds and serves a population of about 332 000 from the western slopes of the Annamite mountains (fi gure 1). We recruited inpatients and outpatients aged 5–49 years who gave written informed consent; were eligible for malaria rapid diagnostic testing or microscopy by Lao national guidelines; had no obvious causes of fever (abscess or severe diarrhoea) that would indicate that there was no need for malaria testing; and whose fever had lasted for 8 days or less with an admission tympanic temperature of 38°C or more. We chose, with restricted resources, to recruit patients aged 5–49 years because 75% of Lao patients with malaria are within this age range. Clinicians at both hospitals recorded clinical features of patients at admission and results of routine laboratory investigations in a case record form. Radiological