The role of vaccination in triggering childhood acute lymphoblastic leukemia (ALL) has been assessed in many studies. The results of these studies were found to be inconsistent. The core consistency and significance of all of these studies is the fact that, all these studies were only case-control based. After Greaves' discovery of the prenatal origin of childhood ALL it is perfectly clear that case-control studies compare genetically quite different populations, i.e. "apples and pears". The only way, this genetic shortcoming of case-control studies to be overcome, is simply to replace it by using cohort studies. Cohort studies, has two great shortcomings, the ethics and the lack of statistically sufficient number of unvaccinated children. The first shortcoming could be overcome by using the retrospective variant of cohort studies, whilst the second one by performing these studies in highly populated countries. The country of choice would be the United States of America.