A sample of 471 enlisted men 183 cm or taller serving in the US Navy, Coast Guard, and Marine Corps was screened for Y-chromosome aneuploidy by use of quinacrine fluorescence of peripheral blood smears. Two 47, XYY males were detected, resulting in a prevalence of .00425 or approximately 1 in 236. The prevalence of 47, XYY males (.00331) in a number of samples of tall, non-institutionalized males is significantly higher than the incidence in newborn males (.00091), indicating that 47, XYY males are disproportionately represented in tall male populations. The 47, XYY males had significantly higher scores than 46, XY males on the Schizophrenia, Schizophrenia + 1K, and Prejudice scales of the Minnesota Multiphasic Personality Inventory and significantly lower scores on the Dominance scale. Since the probability that a randomly selected pair of subjects in the sample had four or more scale scores significantly different from the remainder of the group was greater than .05, it is possible that the differences between the 47, XYY and 46, XY males occurred by chance. On the other hand, one or more of these scales may measure personality dimensions on which non-institutionalized 47, XYY males may, in fact, differ from 46, XY males.