Parents who sit their toddlers in front of the TV could be damaging their child's future learning abilities, US researchers fear.
Viewing, if any, should be age-appropriate, say experts
TV viewing before the age of three was linked to poorer reading and maths skills at the ages of six and seven among the 1,797 children they studied.
The University of Washington findings back the US advice that children under two should not watch any television.
But TV viewing among those aged three to five seemed to aid literacy later.
How much TV?
Some experts argue that educational programmes can foster academic skills in children, however young. They say the key is making sure that the viewing is appropriate for the age of the child.
For young children, programmes should offer opportunities for verbal responses and a balance between familiar and new content, for example, says the National Literacy Trust.
This can actually help language skills, the NLT believes.
However, the Washington University team found each hour of average daily TV viewing before the age of three had a negative effect on scores in mathematics, reading recognition and reading comprehension in later childhood.
Yet, TV viewing among children aged three to five appeared to be beneficial - at least for reading recognition and short-term memory skills - they told Archives of Pediatrics and Adolescent Medicine.
TV has also been blamed for rising childhood obesity and violent behaviour, as well as attention deficit disorder.
How might it damage?
Although the current study authors, Dr Frederick Zimmerman and colleagues, did not look at why too much TV might be harmful, they said there were a couple of possible theories.
One is that children spend less time on other educational and play activities because of the time that they spend watching TV.
The other is that the intense visual and auditory output from TV damages the child's rapidly-developing brain.
Dr Zimmerman's team said: "These results suggest that greater adherence to the American Academy of Pediatric guidelines that children younger than two years not watch television is warranted."
Although there is no such standard advice in the UK, experts agree that carers should limit children's viewing times.
Liz Attenborough of the National Literacy Trust said: "For the most part, we would recommend less than 30 minutes television or video watching for under-three's, preferably with an adult watching at the same time in order to talk about what has been seen."
This is because language develops by talking and interacting with adults, she said.
She added: "We would welcome similar research to be undertaken in the UK, where we enjoy higher quality programming specifically directed at under-three's."
Another study in the same medical journal, by Dr Robert Hancox from the University of Otago, Dunedin, New Zealand, found that children aged five to 11 who watched the most television were the least likely to leave school with qualifications.
Another, also published in the archives journal, of Californian school children aged eight found those with a TV in their bedroom, but no home computer, achieved the worst scores in school achievement tests.
Those without a bedroom TV, but access to a computer, scored the highest.