Clinical studies have shown proguanil (PROG) combined with atovaquone (ATQ) to be an effective and safe antimalarial combination for the treatment of multidrug-resistant falciparum malaria. PROG is a prodrug, which undergoes hepatic metabolism to its pharmacologically active metabolite cycloguanil (CYC). Individuals exhibit genetic polymorphism with respect to PROG, and can be phenotyped as either extensive metabolizers (EMs) or poor metabolizers (PMs) by measuring their PROG/CYC concentration ratio in plasma following PROG/ATQ administration. PMs produce lower plasma concentrations of CYC than EMs and thus may be more susceptible to prophylaxis or treatment failure. Both PROG and CYC potentiate the activity of ATQ in vitro. The antimalarial activity ex vivo of Thai patients' plasma samples obtained from EMs and PMs given concurrent PROG and ATQ was studied using the K1 isolate of Plasmodium falciparum. This isolate is resistant to PROG and CYC, but sensitive to ATQ. Maximum inhibitory dilution profiles of the patients' plasma samples containing PROG and ATQ from EMs and PMs were similar. These findings indicate that differences in plasma drug concentrations between EMs and PMs did not alter the antimalarial activity in vitro against the K1 isolate. The phenotypic status of individuals is not an important issue in the treatment of patients with PROG/ATQ.