Metabolic and biochemical changes during breast carcinogenesis enhance cellular acid production. Extrusion of the acid load from the cancer cells raises intracellular pH, while it decreases extracellular pH creating an inverted pH gradient across the plasma membrane compared to normal cells and promoting cancer cell metabolism, proliferation, migration, and invasion. We investigated the effects of breast carcinogenesis on the mechanisms of cellular pH control using multicellular epithelial organoids freshly isolated from human primary breast carcinomas and matched normal breast tissue. Intracellular pH was measured by fluorescence microscopy, while protein expression was investigated by immunofluorescence imaging and immunoblotting. We found that cellular net acid extrusion increased during human breast carcinogenesis due to enhanced Na(+),HCO3 (-)-cotransport, which created an alkaline shift (~0.3 units of magnitude) in steady-state intracellular pH of human primary breast carcinomas compared to normal breast tissue. Na(+)/H(+)-exchange activity and steady-state intracellular pH in the absence of CO2/HCO3 (-) were practically unaffected by breast carcinogenesis. These effects were evident under both acidic (pH 6.8, representative of the tumor microenvironment) and physiological (pH 7.4) extracellular conditions. Protein expression of the Na(+),HCO3 (-)-cotransporter NBCn1 (SLC4A7), which has been linked to breast cancer susceptibility in multiple genome-wide association studies, was twofold higher in human breast carcinomas compared to matched normal breast tissue. Protein expression of the Na(+)/H(+)-exchanger NHE1 (SLC9A1) was markedly less affected. We propose that upregulated NBCn1 during human breast carcinogenesis contributes to the characteristic acid distribution within human breast carcinomas and thereby plays a pathophysiological role for breast cancer development and progression.