Below are excerpts from veterinary textbooks and medical journals describing the benefits of neutering a male dog or cat:
Neutered dogs do not have testicles, meaning they cannot get testicular cancer. There are 3 common types of testicular tumor: Sertoli cell tumors, Leydig cell tumors, and seminomas.Sertoli cell tumors, ironically, secrete estrogen, a female hormone. Dogs with hyperestrogenism display female physical and sexual traits. Leydig cell tumors can cause hernias and prostatic disease.Fossum, et al. Small Animal Surgery, 2nd Edition, 2002. p660
Neutering a male dog will effectively reduce or eliminate objectionable behaviors such as urine marking, mounting, and roaming. Some dogs show decreased aggression after castration as well. Neilson JC, et al. J Am Vet Med Assoc 211:180, 1997
Benign Prostatic Hyperplasia is an enlargement of the prostate gland that can cause difficulty defecating or urinating. The enlargement is hormonally controlled, and neutering the dog is the definitive cure for this. If a dog is already neutered, he cannot develop BPH. Kraweic, Heflin. Study of Prostatic Disease in Dogs: 177 cases. J Am Vet Med Assoc 100:1119, 1992.
Prostatic abscesses are painful and cause systemic signs. This condition affects mainly non-neutered dogs. Mullen, et al. Results of Surgery and postoperative complications in 92 dogs treated for prostatic absessation by a multiple penrose drain technique. J Am Anim Hosp Assoc 26:369, 1990
Neutering can prevent perianal adenomas and perineal hernias, two painful and unsightly conditions. Fossum, et al. Small Animal Surgery, 2nd Edition, 2002. p619
If a male cat is neutered, fighting decreases by 88%, urine spraying decreases by 87%, and roaming decreases by 92%. Hart BL, CooperLC. Factors relating to urine spraying and fighting in pre-pubertally gonadectomized cats. Journal of American Veterinary Medical Association 184:1255-1258. 1994.
Early neutering can effectively prevent interdog aggression. Voith VL. Intermale Aggression in the dog. Modern Veterinary Practice 61(3):256-258, 1980e.