Surveys of drug resistant falciparum malaria were conducted in several Afghan refugee settlements, distributed over a 700 km range in western Pakistan, during the transmission seasons of 1994 and 1995. Symptomatic malaria patients were recruited by a process of passive case detection at the refugees' basic health units. To facilitate follow-up by local health workers, a modified version of the WHO extended in vivo test was adopted in which blood smears were taken from each subject, and clinical symptoms recorded, at weekly intervals. Resistance to chloroquine and sulfadoxine-pyrimethamine was identified in every settlement. The frequency of chloroquine resistance ranged from 18% to 62%. Resistance occurred mostly as RI, with RII resistance never exceeding 11%. Resistance to sulfadoxine-pyrimethamine occurred at much lower frequencies, ranging from 4% to 25%. There was a resumption of clinical symptoms at the onset of parasite recrudescence in over 90% of cases. The policy of using chloroquine as first-line treatment might be changed in favour of sulfadoxine-pyrimethamine in most camps and areas of western Pakistan. The modified in vivo test was almost as accurate as the normal WHO in vivo test in identifying the grade of resistance, and should prove a useful tool for the monitoring of resistance to common antimalarials by district health services.