We studied the impact of 96 hours of formalin fixation on estrogen receptor (ER), progesterone receptor (PR), and HER2 testing by comparing immunohistochemical results from core biopsy specimens fixed under current American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines with results for corresponding resection samples fixed for 96 hours. Samples enriched with cases showing weak to moderate receptor expression on core biopsy were included in the study. Cases were scored using ASCO/CAP guidelines. Of the 47 cases, only 1 case (2%) showed a qualitative change in result. However, this change was a positive ER result (H score, 1) on the 96-hour fixed resected sample compared with a negative ER result (H score, 0) for the core biopsy. Minimal changes in semiquantitative H scoring were noted for ER and PR that were likely due to tumor heterogeneity and/or intraobserver variability as the variation occurred in both directions. ER, PR, and HER2 immunohistochemical results should be considered valid for cases fixed up to 96 hours.