Beta-thromboglobulin, a platelet-specific protein secreted into the plasma during the release reaction, was measured in 73 women with breast cancer, 10 women with dysplastic disease and 30 healthy female control subjects. 73% of patients with primary operable breast cancer and 65% with metastatic disease had levels above 40 ng/ml (control range 11-38 ng/ml). Among 2-6 months' postoperative patients without evidence of disease at the time of sampling, beta-thromboglobulin was raised in 75% of patients with nodal involvement and in 28% of patients without nodal involvement. Only 1 of 10 patients with benign disease showed abnormal beta-thromboglobulin concentration. Our results show that in a high percentage of breast cancer patients a state of ongoing in vivo platelet activation may be present. Furthermore, beta-thromboglobulin can be regarded as a potential tumor marker in breast cancer.