Six and a half years ago, Richard Scott was 22 and "in perfect health," about to graduate from American University, about to take up competitive bicycling. Then he noticed something odd: His right testicle had grown about twice as big as his left. There was no pain, no specific lump, just a vague swelling and heaviness.

Scott procrastinated for a while, more embarrassed than worried, before finally consulting a doctor. He was referred to a urologist, who immediately suspected a tumor and scheduled Scott for surgery the next day.

Under spinal anesthesia, Scott underwent a radical orchiectomy -- removal of a testicle -- and the pathology report came back grim: malignant tumor.

Today he's fine, working as a social worker for Arlington County, physically active and healthy, married and expecting to father children. He's also helping to get across to young and middle-aged men a message that he was unaware of until a chilling diagnosis drove it home:

*Cancer of the testes -- the male reproductive glands -- is the most common cancer in men 20 to 34. Among cancers, it ranks second in men 35 to 39 and third in men 15 to 19. About 5,000 cases are diagnosed in the United States each year. (Testicular cancer is 4 1/2 times more common in white men than in black men; nobody knows why.)

*If it's caught early, testicular cancer is one of the most successfully treated cancers. The five-year survival rate is 87 percent for all patients, and 96 percent for those whose cancer is promptly detected and has not spread to other organs, according to the American Cancer Society.

*Since there often are no other early symptoms, the best way to detect testicular cancer is for men to examine themselves about once a month. (See box.)

"It's surprising how few people know about it," said Dr. Fran DuRocher, an internist with Group Health Association's Annandale Center. "They've never heard of testicular cancer, or they've never heard of self-examination, and certainly nobody ever taught them to."

Some men, even after discovering an abnormality in a testicle, delay seeking medical attention out of fear, ignorance or apathy. Whatever the reason, it's dangerous.

"A two-month delay could be very serious for a tumor that is spreading," DuRocher said.

Before 1970, the prognosis for men with advanced-stage testicular cancer was poor; more than 90 percent died within five years. Today, with improved treatment by surgery, radiation and drugs, the odds are nearly reversed.

There are two basic types of cancer of the testis: seminoma and nonseminoma. The two differ in cell composition, tendency to spread and pattern of spread -- and are treated differently.

Seminomas are less likely to spread to other parts of the body. They usually are treated with surgery -- removal of the testicle -- and radiation. Some seminoma patients, including those whose cancer has spread beyond the testicles, also receive chemotherapy.

Unlike seminomas, most nonseminomas are not diagnosed before they have spread. Nor are they as susceptible to radiation. Treatment begins with surgery, and if the abdominal lymph nodes show no evidence of the cancer's spread (as in Richard Scott's case), no further treatment is necessary. Patients whose cancer has spread usually receive chemotherapy, including a combination of potent anticancer drugs such as cisplatin, vinblastine and bleomycin.

Although survival rates have risen steadily for testicular cancer, many patients are concerned about the potential side effects of treatment on sexual function and fertility. While the fertility results are mixed, doctors say, much of the concern about sexual function is unwarranted.

Sperm counts often are low in patients with testicular cancer -- even before treatment. In the current Journal of the American Medical Association a leading Danish fertility researcher is quoted as saying that new studies suggest that "in some cases a common genetic or environmental factor is responsible for both the impaired spermatogenesis sperm production and the predisposition to the development of cancer."

But there is no evidence of increased risk of birth defects in children conceived after their fathers were treated for testicular cancer, said Dr. Jorgen G. Berthelsen, senior research fellow at the fertility clinic of the Department of Obstetrics and Gynecology in Copenhagen.

Orchiectomy, or removal of a testicle, does not impair sexual function or fertility. The remaining testicle usually can produce enough hormones and sperm. For patients concerned about the cosmetic loss, an artificial testicle filled with silicon gel can be implanted in the scrotum when the natural testicle is removed, or later.

Removal of abdominal lymph nodes does not affect a man's ability to have erections or orgasms, but it can cause infertility by impairing ejaculation of semen.

Radiation therapy does not affect sexual function, but it does interfere with sperm production. If the rest of the reproductive organs, including the other testicle, are properly shielded, infertility is temporary. "Most patients," a National Cancer Institute report concluded last year, "regain fertility within a matter of months."

Chemotherapy can cause many unpleasant side effects, such as fatigue, nausea and hair loss. These usually are temporary. Some anticancer drugs can also cause infertility, but NCI reports that recent studies show that many patients recover fertility within two or three years.

Richard Scott's cancer was caught in the early stage, before it had spread to other organs. After removal of the testicle, and diagnosis based on detailed laboratory studies of the tumor, he underwent abdominal surgery to remove lymph nodes -- the most likely tissue where the cancer might have spread.

Because of the particular type of cancer he had, and because it was caught so early, Scott had no further treatment. Every three months for the first two years, and twice a year since, his blood is tested for tumor markers -- chemical "red flags" secreted by a tumor. So far, there is no further evidence of cancer.

"I lucked out," he said.

Now 29, Scott is fit and active. He resumed bicycle racing after his surgery and was at one point cycling 400 miles a week. (Home ownership recently cut into his bicycling time.)

Because of a nerve cut during the abdominal operation, Scott cannot ejaculate during orgasm. The semen he produces flows into his bladder and is excreted with urine. But that has no effect on potency or sexual performance.

"The way I look at it," he said, "it's a form of birth control."

If Scott and his wife decide to have children, he said, semen will be taken from his urine -- or, if necessary, withdrawn through a catheter -- for possible artificial insemination.