Surgeons have long been concerned that cancer may be disseminated by shedding of tumour cells into the bloodstream during surgery. Early claims that cancer operations induced an increase in the number of tumour cells shed into the circulation were subsequently discredited, and the issue has remained unresolved. We used immunocytochemistry for cytokeratins to detect tumour cells in effluent blood from breast carcinomas in 18 patients undergoing surgery. Tumour cells were detectable in 6/18 patients during surgery, in only one patient before operation and in none post-operatively (P = 0.025). Circulating cells were associated with vascular invasion within the primary tumour (P = 0.032). No cytokeratin-positive cells were found in blood from ten normal volunteers or four patients undergoing surgery for other breast conditions. These results confirm that cancer surgery in humans results in an increase in the shedding of tumour cells into the circulation. The implications for prognosis and practice should be determined by larger prospective studies.