Controversy seems to follow autism like the tail on a kite. From the earliest descriptions of autism, clinicians questioned whether this disorder was an independent entity. For instance, in 1959, Bender 1 highlighted the imprecision of the term because many distinctly different etiological pathways could converge in the same disorder and concluded that autism was neither an etiological nor clinical entity. In contrast, in 1952 Van Krevelen's skepticism about the existence of autism was reversed when he encountered his first patient who was "as much like those described by Kanner as one raindrop is like another." 2 Others argued whether autism was psychogenic or hardwired in biology and suggested that factors as diverse as mothering 3 or the reticular formation 4 could be the common etiologic denominator. More recently, controversy continues and is reflected in changes in diagnostic criteria and the increasing recognition of heterogeneity that incorporates the word "spectrum" into the diagnosis. With each new prevalence estimate that suggests inexplicable and substantial increases in the number of children diagnosed with the disorder, 5 the urgency to better understand causation is amplified.Although the evidence is already abundant that no relationship exists in the general population between measles, mumps, and rubella (MMR) vaccine receipt and autism spectrum disorder (ASD) risk, 6 immunization rates remain low in certain populations and countries because of this inappropriate belief. Descriptions of autism contain histories of children who seemingly were suddenly affected by a catastrophic developmental event between 1 and 2 years of age-a time in proximity to a scheduled MMR immunization. It made sense knowing this temporal window to ask, "Could it be that, if all of the requisite genetic and other risks are present, MMR can lead to the development of autism?" If so, the population in which there might be such a signal would be families already affected by autism.In this issue of JAMA, Jain and colleagues 7 evaluated 2 questions in their large insurance claims database: does the incidence of ASD differ in younger siblings of affected children who are immunized with MMR vs those who are not? And, for the population as a whole, does the incidence of ASD vary as a function of MMR immunization status? The answer to both questions is no.In the report by Jain et al, 7 of 95 727 children with older siblings who were included in the study, 1929 had an older sibling with ASD and 994 children had ASD diagnosed. The relative risk of ASD at age 2 years was 0.76 (95% CI, 0.49-1.18) for children with older siblings with ASD and 0.91 (95% CI, 0.67-1.20) for children with older siblings without ASD.