Chromogenic in situ hybridization (CISH) has shown the potential to replace fluorescence in situ hybridization (FISH) to determine HER2 gene status. To validate the reliability of CISH, we used 226 consecutive breast carcinomas from 2 institutions and tested CISH and FISH on the same tumor set simultaneously at different test sites. Besides manufacturers' scoring criteria, the new American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines were used to interpret HER2 status. The concordance between CISH and FISH for positive and negative results was 98.5% at site A and 98.6% at site B using the manufacturers' criteria, and 99.0% at site A and 99.1% at site B using the ASCO/CAP criteria. Reproducibility of CISH results was more than 98.0% among 3 sites using the manufacturers' criteria and 100.0% between 2 sites using the ASCO/CAP criteria. Our results confirm that CISH is reliable for HER2 testing per ASCO/CAP guidelines.