A growing stack of studies documents the importance of good day care to children--and not just while they're preschoolers.

The latest addition came last month, when researchers reported that high-quality day care helps children in the long run as well as the short, boosting their school performance at least through second grade.

The six-year ongoing study, known as the Cost, Quality and Outcomes Study, offers what researchers called the first evidence that the quality of preschool day care makes a lasting difference in the performance of children in school.

How well or poorly day care prepares children for school is a hot-button issue, because about half of all women with infants are employed, and most of them return to work before their children are 6 months old. Nearly two-thirds of women with children under 6 years of age now work outside the home, compared with 39 percent in 1975.

An estimated 68 percent of 3-year-olds, 78 percent of 4-year-olds and 84 percent of 5-year-olds, 6.8 million preschoolers in all, regularly receive child care outside the home.

"Thus, the task of fostering children's physical, emotional, social and cognitive development before school entry has shifted away from being solely the responsibility of families to being shared by families and child-care providers," last month's report noted.

Child care is far from the only factor that determines a child's success in elementary school, researchers noted. "We should not hold hopes that high-quality child care will forever erase the major disadvantages some children face as they come to school," the report cautioned. But the study's results underscore the importance of good child care as a foundation that promotes school readiness and school success, said Barbara Willer, deputy executive director of the National Association for the Education of Young Children.

While day care is often thought of as merely a way of taking care of young children when their families aren't available, it is more than that, Willer said. "For children in child care, child care is their preschool."

The study's findings share "one overarching implication," said Yale University's Sharon Lynn Kagan, one of the study's lead researchers. "If Americans want all of our children to be ready for school, we must improve the quality of our child care."

Two of the findings are particularly striking, Kagan said. One is the heightened effect of child-care quality--good and bad--on children who are already at increased risk of failing in school because of their mothers' lack of education. Such children got a bigger boost from good-quality day care and suffered a bigger setback from poor-quality day care, researchers said.

The other key finding is "the sustained effect" of the people who provide the care, Kagan said. Children who had a close relationship with their day-care workers--who got more individual attention--tended to do better in math at least through the second grade. For a 7-year-old, that means the effect had lasted nearly half the child's life. "It's profound," Kagan said.

The study was conducted by a team of researchers--all specialists in early childhood development and education--from four universities: the University of North Carolina at Chapel Hill (UNC), the University of Colorado Health Sciences Center, the University of California at Los Angeles (UCLA) and Yale University. It was funded by the U.S. Department of Education and eight foundations.

Researchers selected more than 800 preschool children in 1993 and tracked them through second grade. They attended about 170 child-care centers in four areas: Los Angeles County; central Colorado; Connecticut's Hartford-New Haven corridor; and central North Carolina. About half of the centers were nonprofit.

The quality of day care at each location was scored on several criteria, including how much individual attention children received from day-care workers; how much they played with other children; and how plentiful and safe the furnishings, toys, books and other materials were. The study corroborated earlier evidence that children tend to do better in centers where they enjoy close relationships with adults and where the staff members focus on the individual needs of the children.

The main findings:

* Children who had attended the higher-quality centers did better in language and math skills. By second grade, those children continued to do better than others in math, although their superiority in language skills tended to decline.

* Children who experienced close relationships with adults in day-care centers had better language and math abilities and better thinking and social skills than other children through second grade.

* For children at higher risk of doing poorly in school--identified as those whose mothers had not finished high school--the link between good quality day care and improved school performance was even stronger.

* Children who attended higher-quality child-care centers tended to have better thinking and social skills in second grade.

* Children who experienced closer relationships with adults and a more positive atmosphere in preschool child care had better relationships with peers--showing less aggressive and disruptive behavior--in second grade.

"Our progress has been far too limited," said Education Secretary Richard W. Riley, who hailed the report's findings as confirmation that "the quality of experiences in the early years matters."

Unfortunately, the number of child-care facilities "that meet high standards and offer employees a livable wage remain[s] woefully inadequate," Riley said. "Every child needs intellectual stimulation, trained caregivers, emotional support, a safe and healthy environment and active parental involvement."

"Children who have poor-quality care enter school not as well prepared," said Ellen Peisner-Feinberg, of UNC, another of the study's main researchers.

The positive influence of good-quality child care carries well into the school years, said Carollee Howes, a developmental psychologist at the UCLA School of Education and Information Studies.

Yale's Kagan compared the investment in preschool child care to an insurance policy for children. Spending more on child care, including higher compensation for teachers, will pay off in improved school performance, she said.

In many areas, Kagan said, "teacher salaries are so low that trained early childhood educators leave the field in unprecedented numbers."

The new report recommends that Congress and the states spend more money on day-care programs, staff training and salaries, and tax credits for working parents, especially those leaving welfare rolls. It concludes that good-quality care is more likely in centers with high staff-to-child ratios, higher levels of staff education, more experienced administrators, higher teacher wages and lower teacher turnover.

Child care in the highest-ranking one-fifth of centers costs about twice as much as care in the lowest one-fifth, said UNC's Richard M. Clifford, one of the study's lead researchers. Experts recommend a maximum of eight to 10 children per adult in day care centers.

Ironically, day-care centers face added pressures in a time of economic boom, researchers said. Not only is demand for day care higher when more parents are employed, but centers also have a harder time retaining experienced staff, because a robust economy enables them to take higher-paying jobs elsewhere.

The first phase of the Cost, Quality and Outcomes Study found that most of the 5 million American children who spend their days in child-care centers are receiving "poor to mediocre" care. Fewer than a quarter of the centers delivered high-quality care, 11 percent were substandard and the rest mediocre, researchers reported in 1995.

Another study, by the National Institute of Child Health and Human Development, found that as a group, children in day care learn to think and use language as well as those cared for at home.

That study, which has followed a diverse national sampling of 1,300 families with young children since 1991, also concluded that day care, by itself, does not harm children's emotional attachment to their mothers.

A follow-up investigation, reported in February, amounted to a "report card" on the nation's day-care centers for very young children. It rated 9 percent of the day care provided for 3-year-olds as "excellent," 30 percent "good," 53 percent "fair" and 8 percent "poor."

The latest study's basic finding--that good-quality day care prepares children better for elementary school--is hardly shocking, said UNC's Clifford.

"It's common sense," Clifford said, "but the reality is that in a lot of places, this common sense isn't put into place."