The records of 2,673 patients randomized according to seven consecutive Eastern Cooperative Oncology Group (ECOG) studies of adjuvant therapy for breast cancer were reviewed for the occurrence of vascular complications. All protocols opened and closed between June 1977 and July 1987. The objectives of the present study were (1) to compare the frequency of vascular complications among patients who received adjuvant therapy for breast cancer with patients on observation, and (2) to estimate the contribution of chemotherapy and hormonal therapy to the occurrence of venous and arterial thrombi. The frequency of thrombosis, both venous and arterial combined, was 5.4% among patients who received adjuvant therapy and was 1.6% among patients on observation (P = .0002). Premenopausal patients who received chemotherapy and tamoxifen had significantly more venous complications than those who received chemotherapy without tamoxifen (2.8% v 0.8%, P = .03). Postmenopausal patients who received tamoxifen and chemotherapy had significantly more venous thrombi than those who received tamoxifen alone (8.0% v 2.3%, P = .03) or those who were observed (8.0% v 0.4%, P less than .0001). Premenopausal patients who received tamoxifen and chemotherapy had a 1.6% frequency of arterial thrombosis, significantly more than patients who received chemotherapy alone (1.6% v 0.0%, P = .004). The frequency of arterial thrombosis among postmenopausal patients was not significantly correlated with adjuvant therapy. In conclusion, patients who received adjuvant therapy for breast cancer had a 5.4% frequency of thromboembolic complications, significantly more than those who were observed. The combination of chemotherapy and tamoxifen was associated with more venous and arterial thromboembolic complications than chemotherapy alone in premenopausal patients and with more venous thrombi than tamoxifen alone among postmenopausal patients.