Bring a medium saucepan of water to a boil. Add the pearl onions and boil over moderately high heat until just tender, about 5 minutes. Drain the onions then cool them under cold water; pat dry. Trim the root end from each onion and slip off the skins.
In a medium bowl, whisk the vinegar with the olive oil. Add the pearl onions, season with salt and pepper and toss. Let stand at room temperature for 1 hour. Add the parsley, toss well and serve.