"We expected to double our seats but this is absolutely fantastic," Agnes Kant, a senior member of the party said. "I am proud the Netherlands wants to move to the left."
Balkenende told supporters on Wednesday night: "We are the biggest party again ... the effort of four years of struggle has been rewarded and that makes me proud."
The failure of any party to win a clear majority means that the Netherlands may have a drawn out process of coalition building before any combination of parties can take power.
Gerrit Zalm, finance minister and member of the free-market Liberal party, Balkenende's current government partner, said: "It's chaos. It is extremely difficult to distil a government out of these results."
Because no combination of left-wing parties have won a majority, the largest parties will have to enter an uncomfortable alliance among parties from opposite sides of the spectrum.
The other alternative is to form a "grand coalition" centring on the two largest parties.
Smaller parties triumph
One of the biggest winners was Geert Wilders who has campaigned on an anti-immigrant message.
The flamboyant campaigner won nine seats in the new parliament - up from only one in the outgoing parliament.
"We are a normal party that wants lower taxes and tougher sentences, but we are also proud of our culture and against the rise of Islam in Dutch society," he said.
"There are enough Muslims in the Netherlands and enough mosques."