BACKGROUND: Clinical symptoms of mixed-species malaria infections have been variously reported as both less severe and more severe than those of single-species infections. METHODS: Oral temperatures were taken from and blood slides were prepared for 2308 adults who presented at outpatient malaria clinics in Tak Province (Thailand) during May-August 1998, May-July 1999, and May-June 2001 with malaria infections diagnosed by 2 expert research microscopists, each of whom was blinded to the other's reports. RESULTS: In each year, temperatures of patients with mixed Plasmodium vivax-Plasmodium falciparum infections were higher than temperatures of patients with P. vivax or P. falciparum infections. In every mixed-species case, P. falciparum parasitemia was higher than P. vivax parasitemia, but patient temperature was not correlated with the parasitemia of either species or with the total parasitemia. CONCLUSIONS: Among adults who self-report to malaria clinics in western Thailand, patients with mixed P. vivax-P. falciparum infections have higher fevers than patients with single-species infections, a distinction that cannot be attributed to differences in parasitemia. This observation warrants more detailed investigations, spanning wider ranges of ages and transmission environments.