When asked why he robbed banks, an astute and success­ ful criminal is said to have replied "Because that's where the money is kept." Why study patients with multiple primary cancers? The answer follows the same practical approach. Because the intensive investigation of such patients is very likely to yield data useful to both the clinical and research on­ cologist. Studies of this problem provide an immediate return for the clinician responsible for the care of cancer patients. With improved forms of therapy, these individuals are enjoying longer periods of survival. One important factor in maintaining increased survival is the early detection and treatment of new primary tumors which may develop. Analyses of multiple primary malignancies serve as a guide to the probable anatomic location of a subsequent primary and help define characteristics of the individual at high risk for multiple primary cancer. But just as treatment may improve the life of the cancer patient, it may also increase the risk of a subsequent malignancy. Studies of multiple primaries provide an efficient means for quantifying potentially harmful effects of current therapeutic modalities.