Abstract. Most patients with suspected malaria do not receive diagnostic confirmation before beginning antimalarial treatment. We investigated the extent to which uncertainty about malaria diagnosis contributes to patient nonadherence to artemether-lumefantrine (AL) treatment through a randomized controlled trial in central Uganda. Among 1,525 patients purchasing a course of AL at private drug shops, we randomly offered 37.6% a free malaria rapid diagnostic test (RDT) and then assessed adherence through home visits 3 days later. Of these subjects, 68.4% tested positive for malaria and 65.8% adhered overall. Patients who tested positive did not have significantly higher odds of adherence than those who were not offered the test (adjusted odds ratio [OR]: 1.07, 95% confidence interval [CI]: 0.734-1.57, P = 0.719). Patients who received a positive malaria test had 0.488 fewer pills remaining than those not offered the test (95% CI: −1.02 to 0.043, P = 0.072). We found that patients who felt relatively healthy by the second day of treatment had lower odds of completing treatment (adjusted OR: 0.532, 95% CI: 0.394-0.719, P < 0.001). Our results suggest that diagnostic testing may not improve artemisinin-based combination therapy adherence unless efforts are made to persuade patients to continue taking the full course of drugs even if symptoms have resolved.