According to sources, Sonia Gandhi held a meeting with senior Congress leaders including her son and party Vice-President Rahul Gandhi before she left.
In her absence, Rahul Gandhi, who is slated to take over the reins of the party, will be leading the party.
According to Congress sources, Gandhi has gone abroad for routine a check up as advised by her doctors.
"She will be back after 3-4 days," said a party source.
In August last year, the Congress chief was taken ill during a road show in Varanasi, Prime Minister Narendra Modi's constituency.
Gandhi had on February 27 visited the Sir Ganga Ram Hospital here for a routine medical check up.
Earlier, Gandhi was twice admitted to the SGRH for her ill-health, during which she underwent a minor shoulder surgery.
Sonia Gandhi has not addressed any election campaign in Uttar Pradesh and instead issued appeal to voters through a statement and video.
The five states where elections were held are Uttar Pradesh, Uttarakhand, Punjab, Goa and Manipur.