Incidence of stroke differs between men and women and it increases nearly exponentially with age. Therefore, assessment of family history of stroke disregarding sex and age of family members results in bias or misclassification. In this study the effects of sex and age on the positivity of the past history were analyzed numerically. Sex- and age-specific proportion of a positive history of stroke among 24,007 family members was obtained from a questionnaire survey of 2,316 high school students. By analyzing the sex- and age-specific proportion with the logistic regression model odds ratios resulting from sex and age difference were estimated. The odds ratio for sex difference was 2.458 (95% confidence interval: 2.067-2.924) and odds ratio for age difference was 1.064 (95% confidence interval: 1.058-1.070). This indicated that a positive history of stroke was 2.458 times higher in male members than in female members of the same age and that a positive history increased by (1.064)y, where y was age difference in years. Potential bias or misclassification resulting from disregarding sex and age can be substantial. Some measures to control for sex and age of family members are required in assessing the family history.