We studied the diagnostic accuracy of a rapid immunochromatographic test (ICT) for detection of IgM against scrub typhus (ST ICT) and an immunoblot test for the detection of IgM against murine typhus (MT IBT) by using admission serum samples from 1,030 febrile patients in Laos. Sensitivity and specificity for the ST ICT determined by using the diagnostic criteria of a four-fold increase in IgM against Orientia tsutsugamushi between paired samples were 23.8% (95% confidence interval [CI] = 15.9-33.3%) and 86.2% (95% CI = 84.1-88.6%), respectively. Sensitivity and specificity for the ST ICT determined by using an admission IgM titer > or = 1:400 were 39.1% (95% CI = 34.1-44.2%) and 99.5% (95% CI = 98.7-99.9%), respectively. Sensitivity and specificity for the MT IBT determined by using the criteria of a four-fold increase in IgM against Rickettsia typhi between paired serum samples were 61.2% (95% CI = 53.7-68.3%) and 86.5% (95% CI = 84.1-88.8%), respectively. Sensitivity and specificity for the MT IBT determined by using an admission IgM titer > or = 1:400 were 54.6% (95% CI = 49.1-60.0%) and 94.1% (95% CI = 92.0-95.7%), respectively. Both assays had relatively good specificity but low sensitivity and thus have limited utility for admission diagnosis.