At least 120 people in Haiti's capital, Port-au-Prince, are being tested for cholera, health officials say.
Doctors have told the BBC the disease is "clinically" present in the city, but there is no official confirmation.
The health ministry says 544 people have died in Haiti's latest cholera outbreak. About 8,000 are being treated in hospitals.
There have been fears cholera would reach the crowded capital since the outbreak began to the north in October.
The water-borne disease has already spread to half of Haiti's 10 regions, and the number of those killed has risen by more than 100 in less than one week.
Authorities feared the outbreak could worsen after Hurricane Tomas brought heavy rains last week which triggered mudslides and flooding.
The storm left 20 people dead, with 36 injured and 11 missing, officials said.
Aid agencies say the main concern is that the flooding could result in the spreading of cholera, with people lacking access to basic sanitation and forced to drink contaminated water.
The Artibonite River to the north of Port-au-Prince, which flooded over the weekend, is believed to be the source of the outbreak.
People living along the river were asked to evacuate the area.
The hurricane passed without destroying the tented camps in and around Port-au-Prince, which house about 1.3 million survivors of January's earthquake.
Aid workers say those living in the tent cities have better access to toilets and clean drinking water than the residents of some of the capital's long-standing slums, says the BBC's Laura Trevelyan in Port-au-Prince.
Health officials were examining at least 120 suspected cases of cholera in the city on Monday, 114 of them in the Cite Soleil slum.
Many of the patients, however, had come to Port-au-Prince from elsewhere in Haiti, including the Artibonite Valley, Gabriel Timothee, the health ministry executive director, told AP news agency.
If the cases are confirmed, the epidemic could threaten an estimated 2.5 to 3 million people in Port-au-Prince.
Cholera itself causes diarrhoea and vomiting, leading to severe dehydration. It can kill quickly but is treated easily through rehydration and antibiotics.