Encephalitis toll rises to 18 in Bihar's Muzaffarpur and Gaya districts, say officials.
Nearly a dozen districts of the state have been put on high alert following the deaths.
The mosquito-borne disease, which claimed nearly 150 lives last year in Bihar, has returned with a vengeance.
Health official Rajendra Prasad has asked civil surgeons in Gaya, Aurangabad, Jehanabad and Arwal districts in particular to be on alert to check the spread of the disease.
A health official in Muzaffarpur said authorities in over half a dozen districts of north Bihar have been told to take steps to check and control the disease.
Eleven children have died in Muzaffarpur and seven in Gaya. Over two dozen children are admitted in hospitals in both places.
Muzaffarpur civil surgeon Gayan Bhusan said the children died after contracting high fever followed by convulsions.
Encephalitis is a condition of acute inflammation of the brain resulting either from a viral infection or when the body's immune system mistakenly attacks the brain tissue.