Estimates of single year maternal age risk rates for Down's Syndrome births to White residents of Ohio over the period 1970-79 are reported. The rates were estimated from birth certificate data, which first necessitated estimating the percentage of reporting of Down's Syndrome on Ohio birth certificates. Using data from cytogenetic laboratories within the state, percentages of Down's Syndrome cases reported were found to be 36.5 (319/875) for White and 33.9 (342/1,010) overall. Final single year maternal age risk rates reported here are corrected for underreporting using these figures. They should be useful for genetic counselors and obstetricians. Comparisons of observed, uncorrected single year maternal incidence rates were made within Ohio 1970-74 vs 1975-79, and between Ohio and upstate New York for various time periods using both a hierarchial log-linear model for multiway cross-tabulations and a weighted least squares solution. All comparisons showed excellent agreement, indicating no evidence for temporal or geographic differences and implying that environmental factors are unimportant in determining single year maternal age incidence rates in Down's Syndrome for the time periods and populations studied.