An Associated Press report shows in eight states, one in 20 public school kindergartners don't have the vaccines required for attendance.
States with the highest exemptions rates were in the West and Upper Midwest. Alaska has the highest opt-out rate at 9 percent, followed by Colorado and Minnesota.
The trend has experts worrying about outbreaks of diseases like measles and whooping cough.
Rules for exemptions vary by state and can include medical, religious or - in some states - philosophical reasons. Some parents are skeptical that vaccines are essential. Others fear vaccines carry their own risks.
The number of shots is also giving some parents pause. By the time most children are 6, they will have been stuck with a needle about two dozen times - with many of those shots given in infancy.
But few serious problems have turned up over years of vaccinations and several studies have shown no link with autism, a theory from the 1990s that has been widely discredited.
Childhood vaccination rates remain high overall, at 90 percent or better for several vaccines, including those for polio, measles, hepatitis B and even chickenpox.
The Associated Press contributed to this story.