Do -1/-1 counters and +1/+1 counters cancel each other out immediately or at end of turn?

When a permanent has both +1/+1 counters and -1/-1 counters on it at the same time, the game removes them in equal pairs as a state-based action. This happens immediately whenever a player would receive priority — it does not wait until end of turn. This is defined in Comprehensive Rule 704.5q.

Specifically, CR 704.5q states: 'If a permanent has both a +1/+1 counter and a -1/-1 counter on it, N +1/+1 counters and N -1/-1 counters are removed from it, where N is the smaller of the number of +1/+1 counters and the number of -1/-1 counters on it.' Both types are removed simultaneously, so a creature with two +1/+1 counters and one -1/-1 counter ends up with one +1/+1 counter and zero -1/-1 counters.

State-based actions (CR 704.3) are checked continuously — before each player receives priority, before a spell or ability resolves, etc. No player can take actions in response to state-based actions being applied; they simply happen automatically as part of the game's bookkeeping.

Concrete example: You control a 2/2 creature with two +1/+1 counters (making it effectively 4/4). An opponent's spell puts two -1/-1 counters on it. As soon as any player would get priority after that spell resolves, state-based actions fire: two +1/+1 counters and two -1/-1 counters are removed simultaneously. The creature is left with zero of either counter and is back to being a 2/2 — no waiting until end of turn.

This immediate cancellation is important for interactions with cards like Hapatra, Vizier of Poisons or Winding Constrictor, since the -1/-1 counters do briefly exist on the creature (triggering abilities that watch for them being placed) before state-based actions erase them.