Uttar Pradesh has 80 seats. BJP's prime ministerial candidate Narendra Modi was leading from the Varanasi constituency.
The party's performance in the state is being largely credited to a campaign led by Amit Shah, close confidante of Modi.
In Bihar, the BJP was leading in 16 of 18 seats from where trends were available. The Congress was leading in just two.
The results from Uttar Pradesh and Bihar seemed to indicate a move away from a culture of identity politics that these states are known for, voting widely in support of the development agenda projected by Modi's campaign.
The BJP was ahead in six seats in Delhi, where it had lost all seven seats in the 2009 elections.