Children who go to playgroups or day-care reduce their risk of the most common type of childhood leukaemia by around 30%, a study estimates.
Researchers reviewed 14 studies involving almost 20,000 children, of which 6,000 developed acute lymphoblastic leukaemia (ALL).
It is thought early infections may help the body fight off the disease.
The study carried out at University of California, Berkeley will be presented to a leukaemia conference in London.
Leukaemia is the most common cancer found in children in the modern industrialised world, affecting around one in 2,000 kids.
'ALL' accounts for more than 8 out of 10 leukaemia cases among children, and most often occurs in those aged between two and five.
Scientists believe that for most types of childhood leukaemia to develop, there must first be a genetic mutation in the womb, followed by a second trigger - such as an infection - during childhood. However, it is also thought that contracting some childhood infections - which are often readily spread in environments such as playgroups where children are in close contact with each other - may prime the immune system against leukaemia. Conversely, if the immune system is not challenged in early life, this is thought to raise the risk of an inappropriate response to subsequent infections, making the development of leukaemia more likely.
The American team found that 12 of the studies suggested some protective effect from social interaction. They found that if several of the weaker studies were weeded out the protective effect could be as high as 40%. However, research showed that children from large families were less likely to benefit than those with fewer brothers and sisters.
Lead researcher Professor Patricia Buffler said: "Combining the results from these studies together provided us with more confidence that the protective effect is real."
Edward Copisarow, of the charity Children with Leukaemia, said: "These findings are important because this is the first time the results of all the relevant studies have been put together and it clearly shows that there is an effect here.
"This is the kind of research that brings us a step closer to understanding the causes of this complex disease and how we can prevent it."