Can I eat rhubarb leaves?
In a word, no.
Rhubarb leaves contain high amounts of oxalic acid, which is poisonous in large doses. Although you'd have to eat a lot of rhubarb leaves to do yourself in, even lesser quantities can be toxic, inducing such unpleasant effects as respiratory failure or coma.
If you're a fan of rhubarb, lop off the leaves and stick to the stalks.