A study of seasonality has been made of birth of individuals with chromosome abnormalities registered in the Danish Cytogenetic Central Register before January 1, 1981. Significant seasonal variation in birth was found for males with Klinefelter's syndrome born before 1946, but not for those born later, and not for any other sex chromosome abnormality. No significant monthly variation was found for any autosomal abnormality, except a significant increase in the frequency of conceptions for Down's syndrome during the first 4 months of the year, using a chi square with 2 degrees of freedom.