Sensitity indices for various serologic tests in the diagnosis of Rocky Mountain spotted fever (RMSF) were determined from RMSF surveillance data collected at the Centers for Disease Control (CDC) during 1981–1984. During these 4 years, a total of 4,141 cases of RMSF in the U.S. were reported to CDC. Case report forms, which include information on dates and results of serologic testing, were received for 3,567 of these cases; 1,774 were laboratory-confirmed according to criteria published previously by CDC. Sensitivity indices for the Weil-Felix (Proteus OX-19, OX-2), indirect fluorescent antibody (IFA), latex agglutination (LA), complement fixation (CF) and indirect hemagglutination (IHA) tests were examined by analyzing cases confirmed by other means, in which data concerning the test in question were available. Analysis of serum pairs for a 4-fold or greater increase in antibody titer or a minimum diagnostic titer indicated high sensitivity indices for IHA (96%) and IFA (94%), but lower sensitivity indices for OX-19 (70%), OX-2 (47%), CF (63%) and LA (71%). IFA and IHA appear to be the most sensitive serologic tests currently in use for the diagnosis of RMSF.