There are more and more studies, including a new study by UC Davis Veterinary School in California, showing an increase in health problems (including certain aggressive cancers) from spaying and neutering, especially when it's done before adulthood.

I have changed my mind about spaying and neutering. I used to recommend, because this is what veterinarians told us, that pets be neutered around 6 months of age, at the onset of puberty. I spayed Chiclet at that age, and neutered Jiggy at that age. If I had the opportunity to do it over, I would have waited longer to spay Chiclet and wouldn't have had her ovaries removed. I probably would have waited to neuter Jiggy as soon as he showed interest in sex. He was several months older, and much larger, than Chiclet and I wouldn't have wanted him to impregnate her. As housemates, there was no way to reliably keep them separate.

Abbreviated article from veterinary journal

An excellent website devoted to spay/neuter

Three Reasons To Reconsider Spay/Neuter

Why I've Had a Change of Heart About Neutering Pets

This "Routine" Neutering Advice May Boost Your Dog's Risk of Cancer and Joint Disease

Early Neutering: We'll Call This Myth Busted...

Dangers of neutering and spaying before puberty

Methods of sterilzation

Announcing a New Safer Alternative to Traditional Spaying

Why These 2 Popular Pet Procedures Might Condemn Your Dog to Endless Suffering…

Why These 2 Popular Pet Procedures Might Condemn Your Dog to Endless Suffering…

Link to Golden Retriever Study

Information about pyometra (uterine disease of females)

DVM Magazine article for veterinarians: Are Intact Dogs Less Likely to Get Cancer?