In 1999, an outbreak involving 188 patients with Legionnaires' disease (LD) occurred at a flower show in the Netherlands. This large outbreak provided the opportunity to evaluate serum antibody tests to assay anti-Legionella pneumophila, since limited data are available on the sensitivity of these tests. The sensitivities of an indirect serotype 1-6 immunofluorescence antibody test (IFAT), a rapid micro-agglutination test (RMAT) IgM serotype 1 antibody assay, and an ELISA to detect IgM and IgG serotype 1-7 antibodies, were evaluated using serum samples from LD patients related to the 1999 outbreak. Sensitivity was calculated using positive culture and/or a positive urinary antigen test as the gold standard in outbreak-related patients with radiographically confirmed pneumonia who fulfilled the epidemiological criteria. The IFAT, RMAT and ELISA showed sensitivities of 61, 44 and 64 %, respectively. The sensitivity of the three tests combined was 67 %. In epidemic situations, however, high standing titres may be included in the laboratory evidence of LD cases. In the study population, high standing titres were found in 16 % of cases. If the presence of high standing antibody titres was added to the criteria of a positive test, the sensitivities of IFAT, RMAT and ELISA were 86, 48 and 75 %, respectively. The sensitivity was 91 % for all tests combined. The higher sensitivity for the combined use of tests is offset by a reduction in specificity to 97·6 %. The results of this study indicate that using a combination of serologic tests in pneumonia patients suspected to have LD does not substantially improve sensitivity. The results suggest that in the microbiological diagnosis of LD, both IFAT and ELISA are reasonably sensitive assays. In an epidemic situation, both tests are highly sensitive, the IFAT more so than the ELISA.