Encephalitis is killing 20 children a week on average in India's most populous state where the death toll has reached 222, health officials said.
The outbreak of "brain fever" began in June and the death toll passed 150 in mid-September in northern Uttar Pradesh.
"Three more children died of encephalitis overnight taking the toll to 222. The majority of the dead were children under 10 years of age," senior health official Umakant Prasad said in Lucknow.
At least 78 children were being treated for the disease in government hospitals, he added.
"The condition of 24 children is serious," added Prasad.
Indian authorities launched a massive drive to inoculate millions of children against Japanese encephalitis, which is endemic in parts of the state, after 1,400 people - most of them children - died in Uttar Pradesh in 2005.
Prasad said the children who were now dying were not among the more than six million who had been given the vaccine.
Like 2005, most of the cases have been reported from impoverished Gorakhpur district, 250 kilometers (150 miles) southeast of Lucknow.
The outbreaks usually begin with the onset of the monsoon rains during June. Mosquitoes carry the disease from pigs to humans.