The late Drew Pearson used to write newspaper columns in the form of letters to a grandchild. I have no grandchild, but I do have a grandmother. I write to her occasionally, scribbling off letters in my head and sending them on their way to where she lives in my imagination. She died more than 20 years ago.

I think of my grandmother from time to time because she was an immigrant and was filled with wonder about her adopted country. Long before Harry Golden used the phrase "Only in America" as a book title, my grandmother said it repeatedly. President Reagan's operation would have had her saying it over and over again.

Can you imagine, grandma, that the world now seems to know everything you can possibly want to know about the president's colon. We know how much of his large intestine was removed and how long the operation took. We know that the tumor was about two inches long, that it took a long time to get to that size and that The New York Times tells us that the removal of the two feet of colon "is not expected to interfere with Mr. Reagan's bowel function." As for The Washington Post, it merely criticized the president's doctors for not operating sooner.

Only in America. In no other country does the leader's large intestine fall into the public domain and do journalists talk as if they have assisted in the operation -- right hemicolectomy, "no touch" procedure, nasogastric tube. In no other country would there be a press room established in the hospital with the doctors standing on stage, answering questions as if they were running for office: What kind of drugs is the president taking? Did they get all the cancer? What are the chances that he does? What are the chances he doesn't? Why did you wait so long to operate? How long, in centimeters, was the tumor?

In the Soviet Union, neither its citizens nor the rest of the world knows until somber classical music is played on the state radio that the leader has died. We never know whether a Soviet leader is sick and, if so, whether the illness is, to use the current clich,e, life-threatening. Russia is a secretive society, but even in other, more open countries clinical details of a leader's health would not be printed in the paper -- and no one would dare question the medical judgment of the doctors chosen to perform the operation. It is probably only in America that doctors are criticized at all.

I have written to my grandmother on less serious matters. I wrote her only recently when Ralph Lauren introduced a faded shirt that cost $2.50 more than one that looks new. I thought she would appreciate a country where people are willing to pay extra for a shirt that looks used. I wanted to tell r that when I called the Lauren company, a vice president there told me that the shirts offered "instant character" and "instant age." Instant age is something I know about. As for character, I'll take any kind.

Woodrow Wilson suffered a stroke and managed to hide his almost total incapacitation from the public. That could not happen anymore, nor could a president get as sick as Franklin Roosevelt clearly was without reporters' asking about his sunken eyes, the color of his skin and the toughest question of all: Is he fit to serve? By Dwight Eisenhower's time such questions had become routine.

President Reagan's operation is an epic piece of Americana. In its own way, it's a celebration of democracy, evidence of the public's right to know almost everything, a suggestion that when it comes to the president and his health we are all family.

In some places overseas, we are once again being criticized for bad taste -- for telling the world more than it wants to know about the president's large intestine. These critics miss the point. The questions -- and the answers -- are nothing more than yet another affirmation of the sense that the presidency is ours and the president is family.

In my imagination, I can see my grandmother taking her favorite chair over to the television set. In coverage that some foreign critics mights consider excessive or in bad taste, she would learn all about the president's health -- his operation, his recovery. And if any of the critics pointed out that things are done differently elsewhere, she would shoot them a look of contempt and offer the ultimate rebuttal. Say it, Grandma.

"Only in America."