Parents are worried about the side-effects of the MMR jab.
Some say their children fell ill with something called autistic spectrum disorder after the injection. Autism is a disability which effects the way people communicate with others. Others say their children developed a bowel disease called Crohn's.
The government did some research to find out whether there was a link between the jab and the illnesses. None was found.
But more research is going on to see if there are more people with autism now than before.
Some parents even think Prime Minister Tony Blair is worried about MMR. He won't say whether his baby son Leo has had the MMR jab and this makes people even more suspicious.