Washington ? Millions of grandparents are acting as primary caregiver to their grandchildren, often because their own sons and daughters are in jail or on drugs.

A Census Bureau report released Thursday found that more than one-third of the 2.4 million grandparents who are primary caregivers to a grandchild lived in a home without the grandchild’s parents.

The report, which looked at data from the 2000 census in greater detail, did not include reasons for that dynamic.

Some grandparents do it because the parents have died, while other parents may be ill, said Sandra Horton, 59, of Lockhart, Texas, who runs a support group and cares for her 12-year-old granddaughter.

But more often than not, it is because a parent is involved with drugs or alcohol, advocacy groups say. Horton called it the “primary, underlying, number one reason.”

These caregiving grandparents typically have not reached retirement age — 64 percent are between age 40 and 59 — so they often find themselves juggling parenting and work.

Data released last year by the bureau showed that the 2.4 million grandparents were responsible for “most of the basic needs” of a grandchild in the home. That is 42 percent of the 5.8 million grandparents living with a grandchild.

In other homes, grandparents may not be the designated main caregiver, but may be helping a single, teenage mother, or may simply be baby-sitting the kids after school, said Amy Goyer of the Grandparents Information Center at AARP, the nation’s largest advocacy group for older Americans.

Horton said she took in granddaughter, Marissa, because the girl’s parents have had drug problems and run-ins with the law. Horton works several part-time jobs to make ends meet.

Horton wants Congress to change guidelines for many existing public assistance programs that make aid available to poor “traditional” families or foster parents, but not custodial grandparents. “Grandparents need respect and recognition for the jobs they do,” she said.