The following example on Legionella pneumophila underlines the considerable potential of this molecular typing method to elucidate connections between an environmental source, in this case a hospital drinking water system, and a case of human illness. At the end of July 2010, the Austrian reference centre for legionella learned about a case of legionella pneumonia in a 70 year old female patient, who had died on July 26. Onset of symptoms had occurred on July 22, admission to hospital C on July 24. A urinary antigen test performed on day 2 of hospitalization was positive for Legionella pneumophila. The patient, who also suffered from neoplasm bronchi with brain metastases, had previously been hospitalized in hospital A for chemotherapy from July 6 to 16, and after spending four days at her private home (from July 16 to 20) - in hospital B for gamma-knife intervention (July 20 to 21). In order to identify the environmental source of infection, efforts were made to obtain a patient isolate for comparison with possible future environmental isolates. A blood culture drawn on July 25 (with a negative result reported by the laboratory of hospital C) was the sole specimen available (autopsy was denied due to religious constraints). Two blood culture bottles were sent to the national reference laboratory and volumes of 0.1 and 0.5 mL were directly plated onto buffered charcoal yeast extract agar with and without glycine, vancomycin, polymycin B and cycloheximide supplementation (Oxoid, Cambridge, UK). After incubating for five days, approximately ten colony forming units (CFU) per ml blood culture medium were gained: Legionella pneumophila serogroup (sg) 1 (ST 81) and Legionella pneumophila sg 3 (ST 93). Double infection is a rare, but not unusual phenomenon, be it in salmonellosis, in tuberculosis or in legionellosis. For the health administrators in charge, an available isolate usually initiates an attempt to identify the source of the patient's infection in order to prevent further fatalities. Legionella is often found in warm and cold water systems of buildings; without proof of clonal identity, the mere demonstration of Legionella pneumophila in a water system cannot be regarded as proof of causal relation with illness. Within the incubation period of 2 to 10 days, the patient had stayed in two hospitals and in her own apartment. The testing of water samples, drawn from the patient's room in hospitals A and B and at her home, demonstrated legionella contamination in all three facilities. Water samples (cold and hot water, mixed) obtained on July 29 from the room occupied by the patient during her stay in hospital A yielded 1 CFU Legionella pneumophila sg 1 (ST 81)/100 mL from the sample "hand washbasin tap" and 7 CFU Legionella pneumophila sg 3 (ST 93)/100 mL from the sample "shower". Water samples (again cold and hot water, mixed) obtained on July 29 from the room occupied by the patient during her stay in hospital B yielded 1 CFU Legionella pneumophila sg 1 (ST 442)/100 mL from the sample "shower". Water samples obtained on August 4 from the patient's home yielded 1-220 CFU Legionella pneumophila sg 10/100 mL. Sequence based typing revealed that hospital A was the causative reservoir. The financial resources necessary to sanitize this contaminated drinking water system would not have been allocated without microbiological proof of a causal connection with human illness.