There have been conflicting reports about whether incidence rates at given maternal ages for Down syndrome (DS) are changing over time. British Columbia provides an excellent opportunity to look at this question as there is 1) nearly complete ascertainment of DS livebirths, 2) a relatively large number of annual births, 3) data on a 20-year time period, 4) data on age-specific fertility rates over this period, and 5) cytogenetic data on all pregnancies terminated through the provincial prenatal diagnosis program since its inception. Data on 856 liveborn DS infants over a 20-year period (1964-1983) were analyzed. Controlling for maternal ages, the data were examined for transient changes in incidence over time and for linear and cyclic trends. No significant (P greater than 0.05) changes in DS incidence were observed over this 20-year period in any maternal age group.