Under the previous definition of an active probable case -- established by Ontario health officials -- Canada had 12 infections from a new cluster discovered in Toronto last week.
But under the newly adopted WHO classification, the number rose to 33, including four who had died soon after the outbreak's discovery.
Health officials switched definitions after coming under fire on Wednesday for possibly under-reporting the illness.
A new increase in cases could put Toronto back on the WHO's travel advisory list weeks after it was removed. Earlier this week, Toronto was reinstated on the list of SARS-affected areas.
The latest outbreak is being traced to a 96-year-old man, who apparently contracted SARS after pelvic surgery at North York General Hospital.
Canadian authorities are particularly worried about the effects of the new outbreak on its already ailing tourism industry.
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