Estimates of mortality rates from age distributions are biased by imprecision in age estimation, even if age estimates are unbiased. I have derived a method for predicting the magnitude of this bias from information on the precision of age determination. Monte Carlo simulations show that bias can be accurately predicted. The commonly used Chapman-Robson mortality estimator is shown to be robust to imprecision in age determination if all age-classes are included. Errors are likely, however, if

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... e or more age-classes are excluded or if other mortality estimators are used. Biases can be corrected if the distribution of ageestimation errors is known.