Antibody titres to L. pneumophila serogroup 1 were followed for 2 years with indirect immunofluorescence test with polyvalent (mostly IgG) conjugate (49 patients) and with specific IgM conjugate (35 patients). Significant titre rises were found in all patients to both Ig-classes. IgM-antibodies were detectable in 13 patients within one week. In all 25 patients that were possible to evaluate in this respect, the maximum titre of both classes was reached within 3 weeks. Although the geometric mean titre for both Ig-classes followed the general pattern for infectious diseases, a sufficient number of patients had persisting high titres of both IgG and IgM making it impossible to designate a reliable value of a single titre to be used as diagnostic evidence of ongoing or recent Legionnaires' Disease.