A multisite study was conducted to assess the performance of the Aperio digital pathology system (Aperio Technologies, Vista, CA) for reading estrogen receptor (ER) and progesterone receptor (PR) slides on a computer monitor. A total of 520 formalin-fixed breast tissue specimens were assayed at 3 clinical sites for ER and PR (260 each). Percentage and average staining intensity of positive nuclei were assessed. At each site, 3 pathologists performed a blinded reading of the glass slides using their microscopes initially and later using digital images on a computer monitor. Comparable percentages of agreements were obtained for manual microscopy (MM) and manual digital slide reading (MDR) (ER, percentage of positive nuclei with cutoffs: MM, 91.3%-99.0%/MDR, 91.3%-100.0%; PR, percentage of positive nuclei with cutoffs: MM, 83.8%-99.0%/MDR, 76.3%-100.0%). Reading ER and PR slides on a computer monitor using the Aperio digital pathology system is equivalent to reading the slides with a conventional light microscope.