The parental age in 77 families of Down syndrome (DS) children with the known origin of extra chromosome 21 and in 12 families of DS children resulting from de novo translocation (more probable than not in 2 meiotic division) was studied. It was shown that when nondisjunction occurred in the 1st meiotic division, both in oogenesis (n = 30) and spermatogenesis (n = 12), mean parental ages and age distributions were different from that of control (400 couples with normal children). The mean age and age distribution were found to differ from control when nondisjunction occurred in the 2nd meiotic division of oogenesis (n = 19). On the basis of our information and the previously published data, lack of the effect of parental age on chromosome segregation in the Ist meiosis may be inferred. It is chromatid disjunction in the 2nd meiosis which is more probably age-dependent. The reasons preventing elucidation of real associations are under debate.