In an attempt to see if the OptiMAL dipstick (Flow Inc., Portland, OR) can be used to monitor antimalarial treatment, a pilot study of 17 patients with Plasmodium falciparum malaria, admitted to the Hospital for Tropical Diseases in London, U.K., was conducted. Sequential, follow-up, blood specimens were obtained from day 1 to day 3, 4 or 5 post-admission. Thin and thick films prepared from these samples were examined for the presence of malarial parasites, and the intensities of parasitaemia were estimated. In addition, each specimen was tested with the OptiMAL dipstick, Rhodamine-123 fluorescence staining and, on specimens collected on day 1 and the last follow-up before discharge, by a PCR-based test. The results showed that OptiMAL has good sensitivity for the initial diagnosis of P. falciparum malaria and also mirrors the decline in viability of the parasites on treatment, giving the potential to follow the efficacy of drug treatment. The results of the PCR-based tests were still positive when the blood film and OptiMAL result were negative. The OptiMAL dipstick compared well with blood-film microscopy for monitoring antimalarial treatment and could be a useful replacement for microscopy to monitor treatment in places where facilities for microscopy are either lacking or inadequate. In developed countries it could be a useful adjunct to blood-film microscopy, and it might permit a reduction in the duration of hospitalization and give an early warning of treatment failure.