I've heard that rhubarb leaves are poisonous, and that you can even die from rhubarb poisoning. Is that true?
The green leaves of the rhubarb contain large amount of oxalic acid and probably toxic anthraquinone glycosides. Eating the leaves can lead to acute poisoning and also death. The anthraquinone glycosides are thought to be the main cause for rhubarb leave poisoning.
On the website of the Canadian Poisonous Plants Information System you find the following about rhubarb:
The stalks are widely used as preserves and are also eaten raw, without problems. The toxic content is much lower in the stalks. Humans have been poisoned after ingesting the leaves. Human poisoning was a particular problem in World War I, when the leaves were recommended as a food source in Britain.
To summarize, the rhubarb stalks are safe, but you should never eat the rhubarb leaves.
According to Rhubarb Info, the leaves are toxic, but on average require a large amount to actually poison you, though it doesn't take nearly as much to make you sick.
Oxalates are contained in all parts of rhubarb plants, especially in the green leaves. There is some evidence that anthraquinone glycosides are also present and may be partly responsible. It is not clear as to the exact source of poisoning from rhubarb, possibly a result of both compounds. The stalks contain low levels of oxalates, so this does not cause problems.