Low levels of lignans, namely enterolactone, have been reported to be associated with an increased risk of breast cancer in the general female population. We assessed, retrospectively, the relationship between serum enterolactone concentrations and the occurrence of breast cancer in women with palpable cysts. The levels of enterolactone in cryopreserved serum aliquots, obtained from 383 women with palpable cysts at the time of their first cyst aspiration, were measured using a time-resolved fluoroimmunoassay (TR-FIA). After a median follow-up time of 6.5 years (range 0.5-12.75 years), 18 women were found to have developed an invasive breast cancer. Median values of serum enterolactone were significantly lower in women who subsequently developed breast cancer: 8.5 nM/l versus 16.0 nM/l: P=0.04. Odd Ratios (OR) for breast cancer were: 0.36 (P=0.03), 0.57 (P=0.3) and 0.38 (P=0.25) for 25th (8 nM/l), 50th (16 nM/l) and 75th (24 nM/l) percentile values, respectively. The receiver operating characteristic (ROC) analysis showed a satisfactory accuracy for enterolactone as a breast cancer risk indicator (area under the curve (AUC)=0.64: P=0.04). Logistic regression analysis confirmed that the enterolactone concentration had a strong protective effect on the breast cancer risk. These findings may have important clinical implications with regard to interventional diet-focused chemo-preventive trials.