Much attention is directed currently to identifying subtypes of cancers that are genetically and clinically distinct. The expectation is that subtyping on the basis of somatic genomic characteristics will supplant traditional pathological subtypes with respect to relevance for targeted therapies and clinical course. Less attention has been paid to the goal of validating subtypes on the basis of the distinctiveness of their etiologies. In this article it is shown that studies of individuals with double primary malignancies provide uniquely valuable information for establishing the etiologic distinctiveness of candidate tumor subtypes. Studies of double primaries have the potential to definitively rank candidate taxonomic systems with respect to their etiological relevance by determining which subtypes are most highly correlated in the double primaries. The concept is illustrated with data from studies of the concordance of estrogen and progestin status in bilateral breast cancers, where it is shown that double primaries are much more likely to be concordant with respect to estrogen receptor (ER) status than for PR status. The high concordance of ER status is consistent with a growing literature demonstrating the etiologic distinctiveness of ER1 and ER2 tumors.Much attention is currently being directed at the goal of classifying cancers into distinct molecular subtypes, using extensive genomic data that distinguishes the somatic characteristics of these subtypes.1,2 It is anticipated that sub-classifications based on molecular characteristics will lead to a better understanding of cancer biology, and better avenues for determining appropriate targeted therapies. 3,4 Efforts to validate the relevance of candidate subtypes have usually focused on establishing clinical distinctiveness on the basis of, say, different survival for patients in the subtypes, or by merely showing that the genomically determined subtypes differ systematically with respect to conventional pathologic criteria. It is reasonable to speculate, however, that subtypes that are genuinely biologically distinct are likely also to possess distinct etiologies. Conventionally, epidemiologists have examined the potential etiologic heterogeneity of cancer subtypes by comparing the candidate subtypes with respect to the presence of known risk factors for the cancer in question. 5 Because it is widely believed that many risk factors for cancer, especially genetic factors, have not yet been identified, this strategy is necessarily limited in scope. The thesis of this article is that optimal subtyping of cancer from an etiologic perspective can be accomplished without the need to know any risk factors, merely by studying the co-occurrence of the subtypes in series of patients with double primary malignancies.The ageing of the population, allied to a gradual increase in survival following cancer has led to a rapid increase in the occurrence of second primary cancers. In fact, over 16% of all cancers reported to the Surveillance Epidemiology and End Results (S...