The Centre for Health Protection is investigating a case of Japanese encephalitis affecting a 54-year-old man.
The man lives in Pu'er, Yunnan and fell ill on July 23. He was admitted to a local hospital the next day.
He returned to Hong Kong on July 27 and was admitted to a private hospital for further treatment.
He tested positive for the disease and is stable.
This is the third case of the disease reported to the centre this year.