Associations of maternal age at birth and subsequent intelligence test scores of children were examined in a series of over 1500 young men from the Netherlands. All subjects were members of 2-child families and were resident in Amsterdam at age 19 yr. Possible confounding by birth order, spacing interval, social class and sex of sibling were considered. Significant correlations between maternal age and child's ability remained in three of the four possible birth order/social class combinations.