In-vivo assessment of the sensitivity of Plasmodium falciparum to chloroquine was carried out in 63 patients in eastern Sudan. Standard triple-dose therapy with chloroquine (25 mg base kg-1 body wt) failed in curing 30 patients. All grades of resistance were demonstrated in the study, confirming that the phenomenon of chloroquine resistance is well established in this area. Factors which may have accelerated the spread of resistant strains include: a drop in the immunity of the local people caused by previous drought, introduction of non-immune refugees, increased transmission following heavy rains, and massive drug pressure. Plasma levels of chloroquine attained in our patients exceeded the therapeutic level and thus ruled out ineffective levels as a possible cause of treatment failure. Moreover, there was no significant difference between chloroquine levels in patients infected with sensitive or resistant strains. As some patients were cured with an additional dose of chloroquine, it is proposed that chloroquine measurement be carried out in patients treated for falciparum malaria with a view to defining new effective levels for semi-immune populations.