Dengue fever and scrub typhus are common infections in Asia that often present as acute febrile illness of unclear etiology. We prospectively evaluated febrile adults presenting to the outpatient department of a hospital in northern Thailand to attempt to identify distinguishing characteristics between the two infections. Fifty-four patients were infected with scrub typhus and 35 were infected with dengue virus. Dengue virus infection was associated with hemorrhagic manifestations, particularly bleeding from the gums, which was reported by 27% of the dengue patients, but by none of the scrub typhus patients (P < 0.001, by Fisher's exact test). A low platelet count (< 140,000/mm3) and low white blood cell count (< 5,000/mm3) were strongly associated with dengue infections: odds ratio = 26.3 (95% confidence interval [CI] = 7.4-93.2) for platelet count and 8.2 (95% CI = 2.6-25.5) for leukocyte count. Prospective evaluations of the usefulness of these simple criteria to differentiate scrub typhus from dengue infection are needed in other areas, particularly where rapid confirmatory diagnostic tests are not available.