All of the babies were born in Ontario and covered by Canada's universal health insurance.
The findings suggest that even with equal access to healthcare, chronically ill infants from poorer families remain at a disadvantage, the researchers report in the Archives of Pediatrics & Adolescent Medicine.
The connection between socioeconomics and health is well known. But according to the authors, this is the first study to follow newborns with complex medical conditions to see how income affects their health, even when they are covered by universal health insurance.
The researchers, led by Dr. Chengning Wang of the Hospital for Sick Children in Toronto, based their findings on data for more than 500,000 infants born in Ontario hospitals between 1996 and 2000.
Just over 2 percent of those newborns had at least one complex chronic condition.
Among these infants, the study found, those from the lowest-income neighborhoods had a 24 percent higher rate of hospitalization during the first year of life than babies from the most affluent neighborhoods.
Their death rate was similarly elevated, though that finding was not statistically significant -- meaning it could have been due to chance.
The reasons for the disparity are not clear, but income-related differences in the home environment -- from unsafe housing to inadequate nutrition -- may be at work, according to the researchers.
In addition, while the Canadian system reduces barriers to healthcare, it does not cover all the costs that families face in caring for a child with chronic health problems, Dr. Chris Feudtner, of Children's Hospital of Philadelphia, notes in an accompanying editorial.
"Universal access will not by itself translate into universal health;" Feudtner writes. "Instead social support programs may be far more important."