First there was "Star Wars." And now, in U.S. District Court here, there is "Diaper Wars."

In the walnut-veneered courtroom of Judge William Hart, more than a dozen lawyers in pin-striped suits are battling it out in a war between two major corporations over the design of disposable diapers.

On one side is Procter & Gamble Co., maker of Luvs disposable diapers. On the other side is Weyerhaeuser Co., maker of a generic diaper, and two of its customers, Montgomery Ward & Co. and J.C. Penney Co.

Diapers? In a federal courtroom?

"Now, this is a serious case," Jerome Lee, attorney for Procter & Gamble, said during his opening statement last week. Indeed, by some estimates, more than $90 million is at stake.

At issue is whether Procter & Gamble has the patent rights to what it calls the best disposable diaper made today and whether Weyerhaeuser copied Procter & Gamble's design to market a disposable diaper that Ward's and Penney's have sold by the millions.

The legal dispute involves the validity of a patent issued in 1975 for a disposable diaper marketed under the name Luvs. In 1981, the last year figures are available, more than 2.3 billion Luvs were sold.

"There are over 3 million babies born in the United States each year," Lee told the jury. "And there are six to eight diaper changes a day. . . . over 20 million disposable diapers sold every day, 365 days a year."

The trial is expected to last four weeks. There are scores of exhibits, and several expert witnesses are to testify about the extensive testing that went into the development of the Luvs diaper.

Lawyers for the defendants contend that the patent issued for the Luvs diaper is invalid because it was not the result of any breakthrough but rather an imitation of previous attempts to design the best diaper. They say the Luvs diaper is similar to a diaper designed and patented by Danielle Talamoni, a French housewife who, after being fed up with existing products, concocted her own diaper in 1971.

Lee explained for the jury how the search for the perfect disposable diaper began in the early 1970s. The diapers then on the market, including Procter & Gamble's Pampers, were just not doing the job, he noted. After a few minutes of wear by an active baby, a disposable diaper would become baggy and leak.

Procter & Gamble officials attacked the problem on a grand scale, according to court documents. They assigned the task of designing the perfect diaper to Kenneth Barclay Buell, a mechanical engineer who had worked on the Gemini space program in the 1960s.

Buell approached the diaper problem with a typical scientist's doggedness, Lee said. He "studied live babies, . . . he made measurements, he studied the movements, the anatomy." Nine months later, Lee said, Buell came up with a "magical combination" of three elements that made for the ideal disposable diaper: an hourglass-shaped, semi-rigid absorbent pad; flexible side flaps, and elastic leg closures that eliminated the "tourniquet effect" of traditional rubber pants but adjusted to prevent leakage.

Then in 1980, Lee said, Weyerhaeuser came out with its diaper. "They decided to make a copy and leave the patent problems to the lawyers," he said.

George McAndrews, lawyer for Weyerhaeuser, told the jury that Talamoni's patent was enough to invalidate Buell's patent. "What Mr. Buell did had been done before by others," he said.

"Once Mrs. Talamoni had taken the three elements and put them together into one integral disposable diaper, then anything else using old art -- the shape of the pad, variances in shape -- were just the work of a technician," McAndrews said.

If there were any doubts of the importance of the diaper patent, Buell dispelled them during his testimony on the witness stand last week. He has worked on history-making spacecraft and has been involved in 10 other patents, but Buell told the jury: "This is one of the greatest things I have ever done, if not the greatest thing."