• In 65 patients evaluated for suspected genetic and/or developmental problems, a roentgenographic study of the kidneys was performed because of the presence of a supernumerary nipple (SNN). Seven of 65 (11%) had a significant renal lesion that included conjoined kidneys in a female with Fanconi's anemia. Four of these patients did not have signs or symptoms suggestive of an underlying urinary tract pathologic condition. These results indicate that an SNN/renal field defect probably exists, although the significance of this association is significantly weaker than that originally proposed by Méhes. One variable influencing this association appears to be racial differences, as evidenced by the absence of renal defects in blacks with an SNN. Based on our findings, the identification of additional minor phenotypic abnormalities may represent an additional mediating variable in this association. However, the discovery of an SNN in an otherwise normal individual, or an individual with a recognizable pattern of human malformation not associated with renal anomalies or central nervous system dysfunction alone, does not appear to be an indication for additional diagnostic studies of the urinary tract.