The Centre for Health Protection is investigating Legionnaires' disease cases involving two men aged 66 and 69.
The 66-year-old man with underlying illnesses lives in Sau Mau Ping Estate, while the 69-year-old with good past health lives in Wong Tai Sin.
A total of 31 Legionnaires' disease cases have been reported so far this year.
There were 66 cases in 2015 and 41 in 2014.