The recent discovery that the extra chromosome in about 30% of cases of 47, trisomy 21 is of paternal origin has revived interest in the possibility of paternal age as a risk factor for a Down syndrome birth, independent of maternal age. Parental age distribution for 611 Down's syndrome 47, +21 cases was studied. The mean paternal age was 0.16 year greater than in the entire population of live births after controlling for maternal age. There was no evidence for a significant paternal age effect at the 0.05 level. For 242 of these Down's syndrome cases, control subjects were selected by rigidly matching in a systematic manner. Paternal age was the variable studied, with maternal age and time and place of birth controlled. There was no statistically significant association between paternal age and Down's syndrome. After adjustment for maternal age, these two studies were not consistent with an increase of paternal age in Down's syndrome.