"Because of its effect on bleeding, it may not be useful in certain critically ill babies," added Van Meurs, a neonatologist at Lucile Packard Children's Hospital at Stanford and a professor of pediatrics at the university's medical school.

The University of Chicago team two years ago was the first to report success with nitric oxide in preemies having severe breathing problems. For its new report, the group looked at how the treatment affected those same children's mental development.

The researchers followed 138 children from the original study — 70 from a control group that did not get the treatment, 68 from the nitric-oxide group. All were assessed by neurologists around their second birthday for development problems, visual impairment, cerebral palsy and hearing loss.

The scientists found that only 24 percent of the treated infants had delayed mental development or a disability, compared with 46 percent who had gotten only oxygen.

"This is the first therapy for premature infants that has demonstrated a significant impact on brain development," said Dr. Michael Schreiber, the study's director and a professor of pediatrics at the University of Chicago.

"Not only does nitric oxide extend life in a large group of premature infants, it also improves the quality of life for the children and their parents," Schreiber said, noting that infants born at 2 pounds typically have only a 50 percent chance of being free of disability or developmental problems at age 2.

"Despite everything we have tried in neonatology, no therapy has ever improved cognitive function," Schreiber said.

The researchers plan to continue tracking the progress of the children from the original study as they move into kindergarten to learn how teachers rate their abilities in a classroom.

The team is also doing more studies to try and understand the mechanism of nitric oxide's effect on preemies' brains, including establishing a formula for how much and how long the substance should be given.

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On the Net: www.nejm.org