Looked-after youngsters are being sent up to 200 miles from Cardiff to Liverpool for care, a council has said.
An inquiry into out-of-county placements showed 244 of 800 cared-for children are based outside of the city.
As a result a Cardiff Council scrutiny committee has suggested building new children's homes.
A council spokesman said 80% are within a 20-mile radius, with parents addresses and foster families accounting for those further afield.
The committee's chairman Lee Bridgeman said some youngsters are being sent to Liverpool, the north of England and north Wales.
"There are young people we understand need to go out of county, for example in cases of sexual exploitation," he said.