The Terai region of Nepal has been known to be heavily malarious since remote times, and it has, therefore, been regarded as uninhabitable by most Nepalese people. The Tharu people, who have been living in the Terai for centuries, were reputed to have an innate resistance to malaria. Following successful control of malaria by the Nepal Malaria Eradication Organization (NMEO), a large and heterogeneous non-Tharu population now inhabits the Terai along with Tharus. By analysing NMEO records, we have found that the prevalence of cases of residual malaria is nearly seven times lower among Tharus compared to sympatric non-Tharus. This difference applies to Plasmodium vivax, which is now much more common, and to Plasmodium falciparum. We suggest that the basis for resistance to malaria in the Tharu people is a genetic factor yet to be identified.