The study was carried out to investigate the status of in vitro susceptibility of Plasmodium falciparum to pyrimethamine (PYR) in multidrug resistant area of the Thai-Myanmar border, the incidence of unregulated use of the combination of PYR with sulfadoxine (Fansidar) in this area and the relevance of pharmacodynamic and pharmacokinetic factors in determining the treatment outcome from the three combination regimens of ART/PYR (1-, 2- and 3-day regimens), in patients with acute uncomplicated falciparum malaria. The majority of patients had baseline PYR concentrations in the range of 1-100 (50.6%) or 100-500 (34.8%) ng/ml, while concentrations of more than 500 ng/ml were found in only 1.1%. All of the isolates exhibited high grade resistance to PYR with the minimum inhibition concentration (MIC) of as high as 10(-5) M. No association was observed between treatment outcome and the presence of baseline plasma PYR concentrations. In addition, lack of association between plasma concentrations during the acute phase (day-1 and -2) and treatment outcome was found.