The objective of this study was to define the conditions under which the body mass of polar bears (Ursus maritimus) can be estimated by morphometry with acceptable accuracy (high precision and low bias). Morphometric and body mass values from 563 polar bears captured and handled in southern Hudson Bay during 1984–86 and 2000–03 were analyzed to determine the effects of sample size and time on the accuracy of estimated body mass (EBM) and to determine the effect of using EBM versus observed body mass (OBM) to calculate body condition index (BCI) values. When sample size was small (≤25), variation around the difference between OBM and EBM was large. However, precision improved markedly with increasing sample size, stabilizing within approximately 3% for sample sizes ≥100. Morphometric–body mass relationships developed for southern Hudson Bay polar bears in the mid-1980s consistently overestimated body masses of bears handled since 2000 by approximately 4%, suggesting relationships within the population had changed over time (increased bias). This was verified by new prediction equations developed for each period that showed the EBM of polar bears captured in 2000–03 is 7–18% less than that for bears captured in the mid-1980s when morphometric values are held constant. Accuracy was reduced when EBM, instead of OBM, was used as a predictor variable for calculation of the BCI. This was caused by both loss of precision and increase in bias as a result of compounding the error associated with the EBM. Although body mass can be estimated accurately by morphometry under specific conditions, we recommend that investigators routinely weigh a proportion of bears captured per field season to ensure and maintain accuracy. The OBM values can be used to both verify the accuracy of EBM values and to calculate BCI values for representative bears.