Paracheck-Pf is a rapid, qualitative immuno-assay for the detection of Plasmodium falciparum-specific histidine-rich protein-2 in samples of human blood. The assay has now been evaluated, against the usual 'gold standard', microscopy, using blood samples from 1655 individuals in five districts of Tanzania, four of which experience frequent malaria outbreaks. The aim was to verify whether Paracheck-Pf could be a reliable tool for the confirmation of malaria outbreaks in such areas. The overall measurements of the assay's performance were good, with a sensitivity of 90.0%, a specificity of 96.6%, a positive predictive value of 88.9%, and a negative predictive value of 97.0% (with an estimated malaria prevalence of 23.3%). There was, however, marked variation between the study districts, the assay's performance being relatively poor where the test had been stored for 12 months at room temperature (23.5+/-3.5 degrees C). The assay was easy to perform in the field and could clearly be a valuable tool in remote areas and in emergency situations, such as the early detection of malaria outbreaks. The cost of the assay (U.S.$0.62/test at the time of the present study) is sufficiently low that its routine use in the confirmation of P. falciparum malaria might also be cost-effective, particularly in areas where there are no facilities for microscopy and/or where the first-line treatment of malaria is based on relatively expensive artemisinin-based combinations.