In this prospective follow-up study the prognostic value of the tumor prostacyclin/thromboxane ratio in human breast carcinoma was investigated. The stable degradation products of prostacyclin and thromboxane (6-keto-PGF1α and TXB2, respectively), were measured by radio-immunoassay in homogenized primary tumours from 29 patients with primary non-metastatic breast cancer. The median follow-up was 43 months (range 24–58 months). Patients with recurrent disease or patients who died of breast cancer had a significantly higher 6-keto-PGF1α/TXB2 ratio than the disease-free survivors (p = 0.018 and p = 0.047, respectively). There was no significant difference in the 6-keto-PGF1α and TXB2 levels. These data indicate that the prostacyclin/thromboxane balance in the tumour might be a prognostic factor in breast cancer. Prostanoids may contribute to metastasis in breast cancer, but the problem is complex because the different prostaglandins have numerous actions that may produce both undesirable and desirable effects.