BACKGROUND Halofantrine, an antimalarial drug used in endemic areas such as the tropics is mainly metabolized by CYP3A4 to the active metabolite N-desbutylhalofantrine. Fluconazole, an antifungal agent and an inhibitor of CYP3A4 is an established part of the therapy in HIV patients, who in turn are prone to malaria in the tropics. This study investigated the effect of fluconazole on the pharmacokinetics of halofantrine after concurrent administration of the two drugs. METHODS The effect of fluconazole on the pharmacokinetics of the antimalarial drug halofantrine was evaluated in 15 healthy volunteers in a Latin Square crossover design. The subjects received a single oral dose of 500 mg halofantrine hydrochloride alone or with 50 mg fluconazole after an overnight fast. Venous blood samples were collected during the next 336 h and analysed by HPLC for halofantrine and its active metabolite N-desbutylhalofantrine. RESULTS Co-administration of fluconazole did not alter the pharmacokinetics of halofantrine significantly with the exception of elimination t(1/2) that was significantly increased by 25% (P < 0.05). In contrast, fluconazole significantly altered the pharmacokinetic parameters of the active metabolite by reducing C(max), AUC and metabolite ratio (N-desbutylhalofantrine/halofantrine) between 35 and 41% (P < 0.05) while increasing t(max) by 50%. The 90% confidence intervals of the ratio of the geometric means (with/without fluconazole) were contained within 80-125% for halofantrine but outside this range for N-desbutylhalofantrine. CONCLUSIONS The decreased plasma concentrations of the metabolite are presumably caused by metabolic inhibition of CYP3A4 by fluconazole. Although the therapeutic consequences of this interaction are not clear caution should be exercised when co-administering both drugs to avoid accumulation and subsequent cardiotoxic effects of halofantrine.