More than one out of every five children in some Arizona counties were without health insurance in 2006 even though they are eligible for nearly free coverage from the state.
And the problem seems to be worse in urban versus rural areas.
The new report from the U.S. Census Bureau comes as Congress is debating whether - and how - to ensure that more people have access to insurance. In the meantime, the burden is left to the states.
But the figures, the most recent available, suggest that, even with highly subsidized premiums, children who are eligible aren't getting the coverage to which they are entitled.
And a just-enacted premium increase for Arizona's program, known as Kids Care, could make the situation even worse as some parents find the higher costs unaffordable.
Everyone earning below the federal poverty level is entitled to free care from the Arizona Health Care Cost Containment System. That figure is currently $18,310 for a family of three.
Kids Care is Arizona's version of the S-CHIP program. Enacted by Congress more than a decade ago, it is designed to insure children of the working poor, defined in Arizona as those in families up to twice the federal poverty level.
The federal government provides $3 for every $1 of state funds to help provide insurance. The monthly premium for parents, at least until last month, ranged from $10 to no more than $35 a month.
Yet even with that, more than 27 percent of Maricopa County children in families below 200 percent of the federal poverty level were uninsured. The figure was slightly less in Yavapai County, about 24 percent in Pinal County and more than 20 percent in Pima, Cochise and Yuma counties.
Dana Naimark, president of the Children's Action Alliance, said she believes the number of children who are eligible but not enrolled is due largely to the lack of knowledge the program exists. And she said some of that blame can be laid at the feet of state lawmakers.
In approving Kids Care more than a decade ago, state lawmakers specifically barred AHCCCS, which runs the program, from contracting with schools to help find children who are eligible.
"They didn't want more people to enroll," Naimark said.
That prohibition has since been repealed. And Jennifer Carusetta, the chief lobbyist for AHCCCS, said her agency even got money to do a six-month "outreach" program last year.
"We've made a lot of progress," Naimark acknowledged. But she said much more needs to be done.
In June 2006 - the time period in the Census Bureau report - there were 58,609 children enrolled in Kids Care. As of this past June, that number had dropped to 53,408.
Carusetta said, though, that could reflect the recession: As more wage earners lose their jobs, they become eligible for the regular AHCCCS program that covers all family members without cost.
But Naimark said many people don't realize they can get nearly free care for their children.
"These are people who are working," she said.
"They've never been on welfare," Naimark continued, and are ineligible for AHCCCS. "They just don't think there's something out there for them."
And she said they won't learn about Kids Care unless the information is available at places they normally go.
"We have not had a sustained effort like that in Arizona," Naimark said. "We've had stops and starts."
Carusetta said her agency was still reviewing the figures to see what conclusions could be drawn about why there are so many children who could be getting health insurance through Kids Care but are not enrolled.
One issue, she said, is that parents might not be able to afford the premiums.
Those in the lowest end of eligibility - between 100 percent and 150 percent of federal poverty level - pay $10 a month to enroll one child and $15 for two or more. That has not changed.
But those between 175 and 200 percent of the poverty level saw their premiums double in June, to $50 a month for a single child and $75 for multiple children.
She also said some children might not be able to meet the requirement to be citizens or legal residents.
Naimark said she believes many of the children are, in fact, here legally, though their parents might be illegal immigrants. She said there is likely a fear factor of those parents to apply for the services to which their children are entitled.
Finally, Carusetta said, there is the question of choice.
"You can't force a person to fill out an application," she said.