Laminin is a noncollagenous constituent of the extracellular matrix (basement membrane). Increased serum concentrations were recorded in patients with a variety of cancers. The clinical usefulness of serum laminin as a marker for breast cancer was investigated in 60 female patients with malignant breast tumors (30 metastatic, 30 non-metastatic). Subjectively healthy age-matched women (n = 30) served as a control group. Laminin was significantly higher in breast cancer patients than in normal controls. Serum laminin levels were also significantly higher in patients with metastasis than in those without metastasis. A positive correlation was observed between serum laminin and the breast cancer-associated antigen CA 15-3 in the tumor patients. The sensitivity and specificity values of laminin for cancer detection at the optimum decision level [mean + 2 SD (1.4 U/ml)] were 75% and 97% respectively, with a 98% positive predictive value, 66% negative predictive value, and 82% diagnostic efficiency. For the detection of metastasis, serum laminin exhibited 77% sensitivity and 100% specificity [best decision level: mean + 2 SD (1.9 U/ml)], with a 100% positive predictive value, 81% negative predictive value, and 88% diagnostic efficiency. The latter specificity and positive predictive value were superior to those obtained with serum CA 15-3. These results suggest that serum determination of laminin could be a useful diagnostic tool in breast cancer and a valuable parameter in the prediction of metastasis.