Detection and typing of Plasmodium vivax by the polymerase chain reaction (PCR) was evaluated in a prospective blinded comparative field study in Thailand. PCR amplification of the circumsporozoite (CS) gene was compared with microscopy for the detection of P. vivax in blood samples from 174 Thai Rangers and 50 malaria-free Bangkok residents. For PCR analysis, filter paper specimens collected by finger prick were randomly processed and blindly interpreted for the presence of the CS gene of P. vivax. The VK210 and VK247 CS variants of P. vivax were detected by specific fluorescein or radiolabeled oligoprobes. Autoradiography with 32P-labeled probes and enhanced chemoluminescent detection with fluorescein-labeled probes identified 91% and 96%, respectively, of 119 microscopically confirmed infections; both systems detected < 100 parasites/microL. Compared with microscopy, the specificity of PCR and radiometric or enhanced chemoluminescent detection was 96% and 90%, respectively. The ease of collection and transport of filter-paper specimens combined with the sensitive and specific detection of allelic genes of P. vivax by PCR suggests that this method may prove to be a valuable tool for epidemiologic and heterogeneity studies of P. vivax.