I affectionately call Nemo my "monster Sheltie" since he measures 19.5 inches at the shoulder, three and a half inches over the breed standard. Due to his big size, he often gets mistaken for a small Collie. On the recommendation of my veterinarian, I had Nemo neutered at 16 weeks old, which I later suspected may have contributed to his extra large stature. Canine sports medicine specialist Dr. Chris Zink DVM has compiled a lot of research  showing that neutering a dog before their growth plates close may cause extra growth and, more importantly, possible health implications.
Researchers at the University of California Davis recently published a study that highlights the need for more work in this area. The team looked at the health records of 759 Golden Retrievers, examining the relationship between neutering and two joint disorders and three cancers (hip dysplasia, cranial cruciate ligament tear, lymphosarcoma, hemangiosarcoma and mast cell tumor). The dogs were classified as intact (not neutered), neutered early (before 12 months age), or neutered late (at or after 12 months age).
Joint disorders and cancer are of particular interest because neutering interrupts the production of certain hormones that influence the closure of bone growth plates and regulation of the estrous cycle in female dogs.
The University of California study found that the rates for all five diseases analyzed were significantly higher in neutered males and females (whether they were neutered early or late) as compared to intact dogs.
Specifically, early neutering was associated with an increase in the occurrence of hip dysplasia (a 100 percent increase!), cranial cruciate ligament tear and lymphosarcoma in males and of cranial cruciate ligament tear in females. Late neutering was associated with the occurrence of mast cell tumors and hemangiosarcoma in females.
The lead investigator, Benjamin Hart, says that it's important to remember that the effects of early and late neutering may vary from breed to breed, since vulnerabilities to various diseases differ.
Knowing this information, it makes a compelling case to get a vasectomy for male dogs (eliminates sperm without effecting testosterone levels) instead of a standard neuter. Unfortunately female dogs don't have an easy alternative.
No matter what, considering how many dogs are neutered early in this country, it's important that more research is done in this area. However, I hope that this study doesn't discourage people from neutering dogs all together. I think we've come a long way in promoting spay/neuter to help control the overpopulation problem (and still have a long way to go). But more research in this area would help us come up with a birth control solution that limits adverse effects on health.
For another view on this study, from a shelter worker, see Shirley Zindler's post.