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In antimalarial chemotherapy, drug resistance is defined as "the ability of a parasite strain to survive and/or multiply despite the administration and absorption of a drug in doses equal to or higher than those usually recommended but within the limits of tolerance of the subject". This official World Health Organization definition, based on clinical and parasitological observations, was established in 1973, when genetics, pharmacology and in vitro culture techniques were still in the early stages of development. Several techniques are currently used to detect drug-resistant Plasmodium falciparum. Several in vivo tests, the traditional gold standard for the detection of drug resistance, have been developed. Classical tests include the 28-day extended test and the 7-day test, interpreted using the S-RI-RII-RIII classification system (S for susceptible and R for resistant, with three degrees of resistance, I to III, depending on parasitological response). These tests cannot be applied in practice, in field situations, and the results do not take into account the clinical condition of the patient, largely because they were designed for use with asymptomatic carriers. These limitations led to the development in 1994 (modified in 1996) of the more practical and simplified 14-day test of therapeutic efficacy. This test classifies the patient's clinical and parasitological response as "adequate clinical response", "late treatment failure" or "early treatment failure". This in vivo test of therapeutic efficacy can be applied in the field with a minimum of health facilities, personnel and other resources. However, true cases of drug resistance may not always be detected by in vivo tests due to pharmacokinetic variations, reinfection, multiple infections, noncompliance or interference with the acquired immune response. The most commonly used reliable in vitro assay, the isotopic microtest, determines the drug concentration at which 50% of parasite growth is inhibited (50% inhibitory concentration IC50). The in vitro assay not only yields quantitative results, it also determines the phenotype of the parasite independently of the immune and physiopathological conditions of the host. However, this in vitro assay requires highly skilled personnel and laboratory equipment. In addition, parasites isolated from patients who have taken medication on their own initiative a few days before consultation usually do not grow in vitro and the interpretation of assay results for patients with multiple infections may be equivocal. One of the major problems with in vitro tests is the determination of the threshold IC50 values that distinguish susceptible from resistant parasites. There are currently no fully validated cut-off points for assessing in vitro resistance. Despite these shortcomings, in vitro tests are of value, particularly if performed in parallel with the in vivo test. Molecular biology has made a major contribution to our understanding of the mechanisms of drug resistance. Discrete point mutations in the genes encoding dihydrofolate reductase and dihydropteroate synthase are strongly associated with resistance in vitro to pyrimethamine and sulfadoxine, respectively. Preliminary results have also suggested that these mutations are responsible for the failure of sulfadoxine-pyrimethamine combination treatment. No causal relationship between discrete polymorphisms in the candidate genes and in vitro chloroquine resistance has yet been established. High-performance liquid chromatography is being increasingly used to determine the plasma concentrations of antimalarial drugs in patients with prophylactic or therapeutic failure, to check that the failure of the treatment is not due to inadequate levels of the drug in the patient. Taking into account all these aspects of resistance to antimalarial drugs we think that the WHO definition of drug resistance is now inadequate. (ABSTRACT TRUNCATED)