The Hawaii Family Study of Cognition provided an opportunity to investigate the frequency and implications of non-agreement, or mismatches, between observed and expected genetic marker phenotypes of husbands, wives, and children. Mismatch data from 68 families in which one or both spouses were known not to be a biological parent were used to determine the rate of undeclared nonparentage in 1,748 families in which conventional relationships were claimed. Two independent approaches gave consistent estimates, suggesting that approximately 2.3% of the 2,839 tested children from these families were probably the result of infidelity, concealed adoption, or another event. About two-thirds of the mismatches detected were probably due to properties of the techniques employed.