A cross-sectional survey was conducted in Bangkok and Samut Sakhon province, central Thailand (Figure 1). A total of 294 foreign migrant workers were enrolled in the study. Each participant was questioned about signs and symptoms of malaria, travel history, and medications taken during the prior two weeks. Between April and May 2008, thick and thin blood films were prepared from fingerpick blood samples of these workers. The study was approved by the Ethics Committee of the Faculty of Medicine Technology at