We studied 10 patients who had a relapse of brucellosis for significant serological changes during relapse. By the Coombs test, the pre-relapse and post-relapse median (range) titers of antibody to Brucella were 1:1120 (1:40-1:10,240) and 1:10,240 (1:40-1:81,920), respectively (P = .0069); by enzyme-linked immunosorbent assay (ELISA) for IgG, these values were 1:2240 (1:60-1:10,240) and 1:10,240 (1:160-1:81,920; P = .0069); by ELISA for IgA, the median titer increased 3.36 times but did not reach statistical significance (P = .0929); by tube agglutination, dithiothreitol (DTT) agglutination, and ELISA for IgM, median titers did not change. Nine patients had a significant titer increase by Coombs test and IgG ELISA, three had a significant increase by tube and DTT agglutination, none had significant increases by IgM ELISA, and one had no significant increase after relapse. Our findings show that for most patients with a relapse of brucellosis, there is an increase in IgG titers, as detected by ELISA and Coombs test, but no change in IgM titers.