In regions with drug-resistant malaria, the ability to rapidly detect or predict treatment failure (TF) soon after a course of standard therapy for Plasmodium falciparum malaria would facilitate the prompt institution of second-line therapy. We thus evaluated longitudinally the ability of the ICT Malaria Pf/Pv immunochromatographic test to predict treatment outcome. Sixty-six Sumbanese Indonesians with uncomplicated falciparum malaria were treated with chloroquine and followed for 28 days by use of 1997 World Health Organization criteria for assessment of therapeutic efficacy of antimalaria drugs. The ICT Pf/Pv testing could be compared with microscopy in approximately half of the patients on each day of follow-up. Although strongly positive histidine rich protein 2 (HRP2) line intensities (equal to or greater than the control band) in convalescence were highly predictive of TF, any degree of positivity for the HRP2 and panmalaria antigens in convalescence was only moderately predictive of TF. Positive predictive values of the HRP2 and panmalaria antigens for TF were 76.9% and 87,0%, respectively, on Day 3, 82,4% and 87.5% on day 7, and 78.9% and 78.9% on Day 14. Negative HRP2 and panmalaria antigens results in convalescence were even less predictive of an adequate clinical response, and false-negative HRP2 and panmalaria antigen test results were found in one-sixth (6 of 37) of recrudescent infection diagnosed by microscopy among patients with late treatment failure. To reliably predic treatment outcome with rapid antigen tests, further development appears necessary to improve sensitivity for viable asexual parasites while avoiding detection of both gametocytes and persistent antigen in convalescence.