I've often read about the human liver's fantastic ability to regrow if lobes are removed during surgery. If this is true, why isn't partial liver removal used to treat some diseases that damage the liver, such as cirrhosis? If a damaged lobe is removed, won't the regrowth be free of the problem, particularly if the damage is 'environmental', such as long-term damage due to alcohol abuse?

I am sure one of the others may be able to give you a more complete answer, but I'll give a brief answer. The answer is that it IS done when possible, but oftentimes the disease has spread beyond what can be reasonably removed by the time it is diagnosed. In many cancers for instance, if caught early enough it is localized enough to only remove the portion of the liver that is diseased. But in diseases like cirrhossis, the disease is often a gradual destruction of the entire liver and there really isn't a stage that can be diagnosed in which it is localized enough to handle by selective removal. Certainly in the gross anatomy labs, I have never seen a cirrhotic liver that would have benefited by removal of a portion because the liver as a whole was too diseased.

Joe is absolutely right - other than metastatic cancer spread, alcoholic cirrhosis, viral hepatitis and inborn errors of metabolism all affect and damage the whole liver. Thus resecting part of the damage helps not at all since the damaged remnant is not able to properly regenerate since it too is damaged.

To complement the answers on a different level, mammals like humans, rats and mice can regenerate their liver after partial hepatectomy. You can easily remove up to two-thirds of the liver in rats and at least half of the liver in humans and it will regenerate. Liver tissue regrowth is not usually identified as proper tissue regeneration because the amputated part is not regenerated. Instead, the remaining tissue expands until the initial mass of the liver is attained. This process is usually defined as compensatory hyperplasia. It was shown that, in normal state, hepatocytes (the main cellular component of the liver) are the ones who divide to restore the lost tissue.