Bernander S, Jacobson K, Lundholm M. A hospital-associated outbreak of Legionnaires' disease caused by Legionella pneumophila serogroups 4 and 10 with a common genetic fingerprinting pattern. APMIS 2004;112:210–7. An outbreak of eight cases of pneumonia caused by Legionella pneumophila non-serogroup 1 (non-sg 1) occurred at a Swedish university hospital in 1993. Including previous and subsequent sporadic cases, the total number of culture-positive patients was 13. Twelve available non-sg1 isolates from patients were compared to 50 environmental water isolates using a monoclonal antibody test for serogrouping and amplified fragment length polymorphism analysis (AFLP). Of the 12 hospital-associated Legionella non-sg 1 patient isolates, 4 were serogrouped as sg 4, 7 as sg 10, and one as sg 6. Using AFLP fingerprinting all serogroup (sg) 4 and 10 isolates were genetically related except for minor variations. Furthermore, sg 4 isolates were identical in AFLP to sg 10 isolates. Patient isolates were also identical to isolates found in the water system of several hospital buildings, but quite unrelated to isolates obtained in a subsequent outbreak at the same hospital caused by L. pneumophila sg 1. Serogroup variations in outbreaks may occur despite a common molecular fingerprinting pattern. Evidently, the L. pneumophila sg 4 and 10 strains were closely related genetically, which raises the question whether this variation in phenotype is due to a genetic event or to a variable phenotypic expression. Genetic fingerprinting should be used in conjunction with serogrouping in epidemiological investigations.