Cohesin is one of the main regulators of sister chromatid separation during the metaphase/anaphase transition. It is a multiprotein complex consisting of 4 core subunits, one of those being the SA2 subunit. SA2 plays the final role in dismantling the cohesion complex from the sister chromatids and also functions in DNA double-strand break repair and gene regulation. There is increasing evidence regarding the involvement of both overexpression and underexpression of cohesin in cancer. Here, we present expression patterns of SA2 in different types of human breast tissue, and the prognostic analysis in the material from breast cancer patients with long-term follow-up. SA2 immunoexpression was evaluated in benign, precancerous, and malignant breast tissue, and was classified into low-intensity or high-intensity groups. The DNA content was determined by image cytometry on breast cancer cell imprints. Prognostic analyses were based on 445 breast cancer patients with upto 20 years' follow-up. SA2 immunoexpression was equally high in both benign and precancerous breast tissue. Instead, 72% of the invasive breast cancers showed deficient SA2 expression. These patients were also associated with an unfavorable outcome as indicated by a 1.6-fold risk of breast cancer death (P=0.0208). The majority (75%) of the patients with low SA2 expression were alive 6.0 years after the diagnosis, whereas the majority of the patients with high SA2 expression survived 17.6 years after the diagnosis. No statistically significant association could be detected between SA2 immunoexpression and DNA aneuploidy. Our results and previous literature indicate that decreased SA2 immunoexpression is associated with malignant breast disease and a particularly unfavorable course of disease.