It's the most common cause of vision problems in children, and only about one in three kids show physical symptoms.
Now, a doctor's invention is helping catch it in just seconds and well before the norm.
Ashton Slowe swims competitively and likes baseball. While he sees himself becoming a professional athlete some day, he can't see much out of one eye.
"The right eye is much weaker than the left eye," he said.
Ashton's dad says the eight-year-old was diagnosed with amblyopia when he was five.
"I would have loved to have had it at a much earlier age," Joseph Slowe said.
Dr. David Hunter's working on that. The ophthalmologist has co-invented the pediatric vision scanner. In just two and a half seconds, the device can catch vision loss or misaligned eyes in kids as young as two.
"Catching it early is essential in order to treat it fully and easily," said Hunter, who is Ophthalmologist-in-Chief at Children's Hospital Boston and professor of Ophthalmology at Harvard Medical School
It's hard to diagnose lazy eye before a child can read an eye chart.
"And it's so frustrating because we know that if just gotten to them three or four years earlier, then we would have been able to fully treat it," Hunter said.
That's what the scanner's blinking smiley face is helping do. Two green lights mean both eyes are fine. One or two red lights mean there's a problem. The prototype was tested on more than 200 kids. It worked better than 96-percent of the time. Dr. Hunter would like to see it become a part of every child's annual check-up.
"Look at their height, their weight, their temperature, their blood pressure and their eye scan," he said.
Dr. Hunter's been working on the pediatric vision scanner for 20 years. It's still in trials now, and work is being done to make it lighter and wireless, but hunter believes it could be in your pediatrician's office sometime in 20-12. If caught in time, lazy eye can usually be corrected with an eye patch or eye drops.