Intracellular Ca2+ dynamics shape malignant behaviors of cancer cells. Whereas previous studies focused on cultured cancer cells, we here used breast organoids and colonic crypts freshly isolated from human and murine surgical biopsies. We performed fluorescence microscopy to evaluate intracellular Ca2+ concentrations in breast and colon cancer tissue with preferential focus on intracellular Ca2+ release in response to purinergic and cholinergic stimuli. Inhibition of the sarco‐/endoplasmic reticulum Ca2+ ATPase with cyclopiazonic acid elicited larger Ca2+ responses in breast cancer tissue, but not in colon cancer tissue, relative to respective normal tissue. The resting intracellular Ca2+ concentration was elevated, and ATP, UTP and acetylcholine induced strongly augmented intracellular Ca2+ responses in breast cancer tissue compared with normal breast tissue. In contrast, resting intracellular Ca2+ levels and acetylcholine‐induced increases in intracellular Ca2+ concentrations were unaffected and ATP‐ and UTP‐induced Ca2+ responses were smaller in colon cancer tissue compared with normal colon tissue. In accordance with the amplified Ca2+ responses, ATP and UTP substantially increased proliferative activity—evaluated by bromodeoxyuridine incorporation—in breast cancer tissue, whereas the effect was minimal in normal breast tissue. ATP caused cell death—identified with ethidium homodimer‐1 staining—in breast cancer tissue only at concentrations above the expected pathophysiological range. We conclude that intracellular Ca2+ responses are amplified in breast cancer tissue, but not in colon cancer tissue, and that nucleotide signaling stimulates breast cancer cell proliferation within the extracellular concentration range typical for solid cancer tissue.