The use of standard external measurements is pervasive in mammalogy, due in part to their applicability to both living and dead animals. Nevertheless, comparisons of measurements made between animals in different pre- and postmortem body states may be problematic. To investigate the impact of body state on standard measurements and their associated variances, we took 5 external measurements on 70 Mus musculus while living and in 3 postmortem body states (primary flaccidity, rigor mortis, and secondary flaccidity). Total length, tail length, and hind-foot length were significantly longer in states of primary and secondary flaccidity than when measured on live individuals or those in rigor mortis. Ear length increased from living to primary flaccidity, after which it decreased. Weight decreased between each postmortem body state, likely as an artifact of desiccation. Variance was always greater for measurements taken in the living state than during postmortem body states. Irrespective of body state, variance was particularly high for ear length and hind-foot length, which are prone to observer bias and should be used with caution. Additional tests using field-collected data from populations of Peromyscus leucopus and Peromyscus maniculatus confirmed our lab-based results from M. musculus. External measurements taken during a postmortem state should not be compared to measurements from live animals and could lead to incorrect species identification. Additionally, comparisons among individuals measured in different postmortem body states may confound efforts to assess changes in body size over space or time.