The indirect haemagglutination (IHA) test using sensitized turkey erythrocytes and the indirect immunofluorescence assay (IgM-IFA) was confirmed to be sensitive in the detection of a recent or current Pseudomonas pseudomallei infection in 19 culture-confirmed Singapore melioidosis patients. All were found to have antibody titres from 4 to 32768 in the IHA test and 10 to 320 in the IgM-IFA test. When these tests were employed on sera from 16 immigrant Thai construction workers who died of sudden unexplained death syndrome (SUDS) and 73 healthy Thai fellow workers, 93.8% and 68.8% of SUDS cases had IHA titre of greater than or equal to 4 and IgM-IFA titre of greater than or equal to 10 respectively, in contrast to 39.7% and 12.3% found among healthy Thai workers. These data indicate that at the time of death, most of the SUDS patients had an active infection with P. pseudomallei, possibly resulting from reactivation of a latent infection. The aetiological role of P. pseudomallei as the major cause of SUDS is discussed.