Objective:Breast carcinoma has no longer been considered as a single and standalone disease. Its subtypes have been known to vary in terms of risk factors, natural histories, and responses to therapies. In particular, intrinsic molecular subtypes based on St. Gallen International Expert Consensus 2013 have classified breast carcinoma into luminal A, luminal B, HER2+, and triple-negative, depending on the expression of ER, PgR, HER2, and Ki-67. Research on intrinsic molecular subtypes of breast carcinoma in Indonesia, however, are rarely conducted, which then triggers the intention to conduct this study. Methods:In this work, a retrospective study was conducted on 92 formalin-fixed paraffin-embedded samples of invasive ductal breast carcinoma patients. These samples were from patients at Abdul Wahab Sjahranie County General Hospital Samarinda, East Kalimantan, Indonesia, in 2016. Next, immunohistochemical staining using anti-ER, PgR, HER2, and Ki-67 antibodies was applied to classify intrinsic molecular subtypes. Then, an association between clinical and immunohistochemical factors with intrinsic molecular subtypes of breast carcinoma were analyzed using Chi-square test. Results:Looking at results of the retrospective study, luminal B was discovered as the most common intrinsic molecular subtypes of breast carcinoma (42.39%) in East Kalimantan, Indonesia. The next ranks of breast carcinoma subtypes in the region included HER2+ (39.13%), triple-negative (10.87%), and luminal A (7.61%). In fact, there was a significant association between age (p = 0.019) with intrinsic molecular subtypes of breast carcinoma. Conclusion:The study found luminal B as the most common intrinsic molecular subtypes of Indonesian breast carcinoma in the region under investigation. In the future, the higher positivity rate of luminal B in breast carcinoma patients compared to prior studies would require further investigations.