GENEVA, JAN. 8 -- The number of AIDS cases reported worldwide jumped 56 percent last year to more than 73,000, but the actual total may be as high as 150,000, the World Health Organization reported today.

The cumulative total of cases since reporting began in 1979 increased to 73,670 by Dec. 30, compared to 47,201 at the beginning of last year, the Geneva-based organization said.

The United States had the most reported cases last year -- 48,139 -- or about two-thirds of the worldwide total of 73,670. France was second with 2,523 cases.

Acquired immune deficiency syndrome strips the body of its natural defense system against diseases. There is no known cure against the deadly virus. WHO said more than half of all the cases have resulted in death.

WHO spokesmen cautioned that the 56 percent increase last year was due not only to a continued spread of the disease but also in large part to vastly improved detecting and reporting in African and other Third World countries.

The known total also could amount to less than one-half of all cases, WHO spokesmen said.

"The figures are somewhat misleading because of differences in reporting techniques, and a more accurate figure would be between 100,000 and 150,000 cases worldwide," Dr. Jonathan Mann, head of a special WHO task force on AIDS, wrote regarding the latest figures.

Mann and other medical experts have been particularly concerned with the spread of AIDS in Africa, where officials fear the detected and reported cases are only a fraction of the actual number.

Known cases in Africa have been doubling annually since 1984 and a substantial share of WHO's funds have been spent on improving the reporting system there.

The first 14 cases of AIDS in the United States were detected in 1979.

Despite deep concern over the situation, WHO has rejected proposals for any international "AIDS-free" certificates or other controls for travelers.