There is conflicting evidence as to whether the secondary sex ratio in humans decreases with paternal age. Such an age effect could be caused by an altered frequency in the production of X-chromosome and Y-chromosome-bearing sperm as a man ages. To study this possibility we analysed 9,225 sperm karyotypes from 143 men aged 21-55 years. Human pronuclear sperm chromosome complements were obtained after fusion with golden hamster oocytes. The percentage of X- and Y-chromosome complements was not significantly related to donor age.