When his filly Landaluce began displaying unmistakable signs of greatness this summer, trainer Wayne Lukas was already contemplating what her place in racing history might be.

"I've never had a horse stun me like this one did," Lukas said. "I didn't have the opportunity to see Ruffian, but a lot of people who saw them both run say this filly reminds them a lot of her."

Lukas could not have imagined at the time how terribly apt the comparison with Ruffian would be. When Landaluce died on Sunday morning, after a week-long illness, thoroughbred racing suffered its most tragic, premature loss since the day in 1975 when Ruffian shattered her leg at Belmont Park.

But at least Ruffian had had a chance to show how brilliant she was, to race as a 3-year-old and win the triple crown series for members of her sex. Landaluce didn't even have that opportunity. She ran only five times, never got to compete in a nationally important stakes race and never got broad public recognition. She did just enough to raise the tantalizing possibility that she could conceivably be the greatest filly who ever lived.

Certainly, few horses have ever delivered a performance as electrifying as Landaluce's victory in the Hollywood Lassie Stakes in July. After dueling for the lead in a seemingly suicidal 43 4/5 seconds for the half-mile, she turned into the stretch with a two-length lead. And then she proceeded to run away from the field, winning by 21 lengths in a record-shattering 1:08 flat. "It was the most incredible performance I've ever seen," Lukas said.

That victory made Landaluce an instant star to California racing fans, but she had many challenges ahead that would truly test her abilities. She was going to have to show, first, that she could carry her speed over a distance; her lone race beyond a mile had been her least impressive. She was going to race against the country's top 2-year-old colt, Copelan, on Dec. 12. And then Lukas was going to look ahead to her 3-year-old season and a possible bid for the Triple Crown. Landaluce was going to be the great horse racing story of 1983.

So formidable was Landaluce that it still seems inconceivable she could be struck down by what had initially seemed to be a routine ailment. A virus had affected many of the horses stabled at Santa Anita, and Landaluce contracted a respiratory infection early in the week. She ran a fever and suffered from some mild intestinal disorders. But the infection produced a blood clot on the filly's lungs, and the veterinarian who performed the autopsy said that was the cause of death. The combination of circumstances that caused her death, he added, "are not common, but they do occur."

For Lukas, the filly's death was a recurrence of a nightmare. Just last year he had been training a brilliant 2-year-old colt, Stalwart, who looked as if he might be the most gifted member of his generation. But an injury abruptly ended his career, and Lukas said, "The experience was absolutely devastating."

When Landaluce came along this summer, it seemed that this was fate's way of evening the score. Instead, her abortive career only reconfirmed how cruel the sport can be.