The city agency said it has received reports of 23 cases of the respiratory disease and that 22 of the patients have been hospitalized. There have been no deaths reported so far.
But officials said it's a higher number than expected at this time of year in Toronto.
Officials did say that most of the patients sent to hospital had other underlying medical problems.
Legionnaires' disease is caused by water-borne bacteria and can often be found in moist areas such as air-conditioning systems, water pipes and showerheads.