During an epidemic of Plasmodium falciparum malaria in Chogoria, Kenya, P. falciparum DNA was collected from 24 cases of severe malaria admitted to hospital for parenteral quinine treatment. These patients had all failed first- (chloroquine) and second-line (sulfadoxine-pyrimethamine or amodiaquine) drug treatments. Twenty-two (92%) of the 24 patients sampled carried parasites with the Asn 86 Tyr point mutation in the pfmdr1 gene (chromosome 5), 20 (83%) had an Asp 1246 Tyr mutation and 18 (82%) had both of these mutations. These alleles are both reported to be associated with chloroquine-resistance. Polymorphisms in the cg2 gene (chromosome 7) are also associated with chloroquine resistance, and 18 (75%) of the 24 parasite samples each had the cg2 and pfmdr1 polymorphisms. These 18 samples also had the mutations associated with resistance to pyrimethamine and sulfadoxine: Asn 51 Ile , Cys 59 Arg and Ser 108 Asn of gene dhfr (chromosome 4) and Ala 437 Gly and Lys 540 Glu of dhps (chromosome 8), respectively. Genotyping of the parasites from all 24 patients revealed extensive diversity in the sequences for the merozoite surface antigens ( MSA-1 and MSA-2 ) and the glutamate-rich protein ( GLURP ) and indicated that each sample contained more than one parasite clone. Although samples from non-admitted malaria cases were not available, it appears that drug resistance may have played an important role in the development of severe malaria in this epidemic.