Surprisingly, little is known about host factors in cases of bladder carcinoma. We investigated 2 families prone to transitional cell carcinoma of the bladder. A high degree of pathology verification of cancer of all anatomic sites and a meticulous recording of genealogy, associated diseases and environmental exposures, when known, have allowed a more cogent appraisal of cancer etiology. It is reasonable to assume that members of the subject families may be more susceptible to variable carcinogenic exposures, a concept that is in accord with a genetic-environmental interaction hypothesis for cancer etiology. In addition to increased surveillance of high risk patients for earlier detection of bladder cancer, cancer control measures also should take into consideration preventive programs directed toward the avoidance of known carcinogenic exposures, such as cigarette smoking in high risk relatives of cancer-affected probands. We propose that the etiology of familial bladder cancer may be complex, involving possible other associated malignant neoplasms and/or certain non-neoplastic disorders, in addition to specific carcinogenic exposures. There is a serious need for the detailed reporting of families prone to bladder cancer wherein all of these potentially important associated factors are considered, so that a fuller appraisal of etiology might be achieved.