Although the effect of maternal age as a risk factor for Down syndrome (DS) is well known, the role of paternal age in the cause of DS has not been clearly established. To investigate this phenomenon we conducted a case-control study between July 1989 and February 1990. The cases were 318 children and teenagers with DS studied at the Specialized Educational Institutions of Lima City, Perú. They were paired with 1,196 control individuals that were selected from the birth records of 2 general hospitals of the city. For each case we tried to obtain 4 controls, paired by their date of birth, sex, and maternal age. The means of paternal age in the 2 groups were compared, first globally and then by groups of maternal age (< 21 years, 21-29 years, 30-34 years, 35-39 years and > 39 years). None of the comparisons gave a statistically significant difference between the 2 groups, using either the Student t-test or the Mann-Whitney U-test. The results obtained in this study give no evidence that paternal age can be considered a risk factor for the conception of a child with DS.