Despite uncertainties in medicine, attempts to study physicians' tolerance for uncertainty have been few, and limited by the measurement instruments available. This paper describes development of a modified tolerance for ambiguity (TFA) scale, and correlates it with several physician characteristics and reported behaviors. Eighteen TFA items were included in a national survey of physicians' knowledge and attitudes about genetic testing. Sixty-five percent (n = 1,140) of 1,759 obstetricians, pediatricians, internists, family practitioners, and psychiatrists responded. After psychometric analyses, the scale was reduced to 7 items, demonstrating an acceptable reliability (Cronbach's alpha = .75). TFA was higher among psychiatrists than other specialties, among those who were older when they graduated from medical school, and among those willing to offer a new low-cost, accurate predictive test when none of their colleagues do. TFA was lower among those who indicated that attendance at religious services was important, among those who would make a recommendation to their patients regarding pregnancy termination after prenatal diagnosis, and among those who would withhold negative genetic test results. Future research is needed on the scale itself, and to assess factors affecting TFA, such as its susceptibility to modification, and its potential association with clinical practice in other areas of medicine that are characterized by ambiguity.