More than 20 million Chinese children are living with grandparents or other relations after their parents left home to find work, an official report said.
China's reforms have seen millions of people become migrants
The report, quoted in the China Daily, said some of the children suffered from loneliness and poor performance.
Their parents are among 150 million Chinese thought to work far from home, attracted by rapid economic growth in richer provinces.
Schooling is the main reason to leave children behind, analysts say.
Migrants have to pay school fees if they take children with them.
The report, prepared by the China National Children and Teenagers' Working Committee, found that in some poorer provinces, one in five children were living without their parents, and the proportion was increasing.
One recent survey, from Anhui province, found that such children had little communication with their parents.
The survey found that most migrants talked to their children by phone once or twice a month, and returned home only once a year, during Spring Festival.