Summary: Chloroquine (CQ) and sulphadoxine/pyrimethamine (SP) are two first-line antimalarials used under the existing Indian National Drug Policy in the north-eastern region of India bordering several countries including Myanmar. Although widespread resistance to antimalarials in Plasmodium falciparum has been reported from western Myanmar, information from the Indian side of the border is scarce. We studied the therapeutic response to CQ and SP at four sites in Changlang and Lohit, two administrative districts of Arunachal Pradesh bordering Myanmar. We monitored uncomplicated falciparum malaria patients after treatment with standard regimens of CQ and SP for 28 days following the revised in-vivo protocol of the World Health Organization. A total of 236 patients, 95 in the CQ group and 141 in the SP group, participated. We recorded 23.8% early treatment failures to CQ and 14.1% to SP; late clinical failures of 14.3 and 12.6%; late parasitological failures of 10.7 and 8.1% and adequate clinical and parasitological responses of 51.2 and 65.2%, respectively. The significantly different treatment failure rates seen in Chowkham (furthest from Indo-Myanmar border) and Jairampur/Nampong (nearest to Indo-Myanmar border) for chloroquine (Cox proportion hazard ratio 9.1, P < 0.0001) and SP (Cox proportion hazard ratio 7.35, P ? 0.001) denote a non-response gradient to the two antimalarials extending from the international border. The gradient is probably indicative of the direction of movement of the drug-resistant P. falciparum parasite. The utility of chloroquine as the first-line drug under the present National Drug Policy in these areas needs reconsideration... Keywords: antimalarial, drug resistance, in-vivo sensitivity, border area malaria, chloroquine, sulphadoxine/pyrimethamine, therapeutic failure