For 25 years, Jovanka Broz was constantly by the side of Yugoslav President Tito, as wife, nursemaid, hostess, and trusted confidante. For the last three months she has been a non-person, never seen in public, ignored by the official press.

The disappearance from public view of Madame Broz, 52, who first met Marshal Tito while serving as a young partisan during World War II, has become a major talking point in Yugoslavia.

When they met, she was 20, a stunning beauty with shiny locks of thick black hair. He was 53, supreme commander of the Yugoslav army and leader of the Communist government. Her official biography states that she was working in the marshal's office until their marriage in 1952.

Reasons for her prolonged absence this fall put forward by diplomats and officials have ranged from a serious illness to a private quarrel with political overtones.

Apart from the human interest in the mystery of what has become of her, there could be some political significance as well. Jovanka, as she is affectionately called by Yugoslavs, was one of a small group of people who formed a kind of kitchen cabinet around the 85-year-old president. Its other members included Edvard Kardelj, the Slovene leader just ending a tour of the United States, and Stane Dolane, the secretary of the Communist Party's executive committee.

In recent years, it is believed that Tito has come to rely increasingly on this inner circle for advice. Many of his old comrades who fought with him against Nazi occupation are either dead, discredited or retired.

Of all the rumors now circulating in Belgrade about Madame Broz's vanishing act, a few hard facts stand out:

The last time she was seen in public was on June 14 at a reception for the Norwegian prime minister.

Contrary to custom, she did not accompany the president on his recent journey to the Soviet Union, North Korea, and China. Nor did she appear in public to welcome him home, as did half a million other residents of Belgrade.

A separate program was arranged for her during President Tito's planned visit to France next week. French officials have now been told that she will not be coming.

In the last few months, there have been a number of significant changes in Tito's private office. His chief of cabinet, Mirko Milutinovic, has been named as ambassador to Pakistan and his foreign affairs adviser is going to India.

The original reason advanced for Madame Broz's absence was illness. She was rumored to be undergoing medical treatment for diabetes in the Montenegrin resort of Igalo where Tito was treated for sciatica. Another rumor said she was in Switzerland.

Privately, many senior diplomats now tend to belive there has been some kind of private dispute between the couple.

According to one party source, Tito's failure to take his wife to China was a clear sign that he wished to disassociate himself from her. This version has been supported by a number of clues that may or may not be important.

These include the perhaps trivial case of the disappearing poodle. Tito and his wife were often photographed together walking their two white poodles in the garden of one of their residences. Yet, on a recent provincial tour, the president was seen to be accompanied by only one poodle.

Government officials have refused to commet publicly on the mystery. "Madame Broz is a private person without political influence. This is not a matter of public interest" was how the official spokeswoman replied to inquiries yesterday.

Asked whether Madame Broz had been ill recently, she said: "I have no information on that."