Legionella pneumophila clinical isolates are mostly serogroup1(SG1) and subgroups with MAb3/1. In contrast, environmental isolates have a low number of SG1 and most is MAb3/1 negative subgroups although a primary place for Legionella infection was environment. This study aimed why MAb3/1 positive SG1 subgroups concentrate on patients by epidemiological study based on molecular genetic technique and whole genome analysis.