Pump it up

[TJKoer]

So I have a creature with undying and I pump it up, then it dies, does it come back pumped still since it would last until end of turn and it is the same creature.

[BlackJester]

No.

If a creature leaves the battlefield and returns, the game treats it like a new creature.  (unless an effect says otherwise)

[Thattallguy]

Also if you pumped the creature with undying up with something that gives it +1+1 counters it won't come back but if it was pumped up with a spell or ability that gives creatures +1+1 than it will come back.

[cltrn81]

Also if you pumped the creature with undying up with something that gives it +1+1 counters it won't come back but if it was pumped up with a spell or ability that gives creatures +1+1 than it will come back.

Right the undying ability only cares if the creature had a +1/1 counter on it.  So an undying creature with no +1/1 counters on it but was pumped with {giant growth} will still return if it dies.