introduction: information on the relationship between treatment failure in malaria and factors of the host (nutritional status, phenotype and genotype of cytochrome cyp450) involved in the metabolism of antimalarials is scarce. objective: to explore whether treatment failure of mefloquine administered to patients with noncomplicated falciparum malaria can be explained in terms of the patient？s nutritional status and the cyp3a4 phenotype and genotype. materials and methods: non-matched case-control study. patients were adult males and females, inhabitants of turbo and el bagre (antioquia, colombia). results: the therapeutic response was assessed in 46 patients, and there were only three failures (6.5%); due to the rare occurrence of therapeutic failure (n = 3/46), results are presented in a descriptive way for the 46 patients. the dextrometorphan/3-methoxymorphinan ratio was 0.39 (median); 20% of the patients were slow metabolizers. the blood concentrations of mefloquine at 24 hours (c24h) and at day 14 (cd14) were (median) 1.363 ± 397 ng/ml and 978 ± 106 ng/ml, respectively. all 46 patients had the wild cyp3a4*2 allele. conclusion: we were unable to assess in depth the relationship between the response to mefloquine, on the one hand and, on the other, cyp450 activity and nutritional status. however, there were findings that justify the assessment and control of the characteristics of the host in subsequent studies of antimalarial pharmacokinetics.