Considering the current need to improve cost-effectiveness in cancer patient management, a prospective study was undertaken in order to define the optimal combination of bone scan and tumour marker assays in breast and lung cancer strategies, as has been done in the case of prostate cancer. All patients with breast or lung cancer referred to the Nuclear Medicine Department of the Grenoble Teaching Hospital between December 1995 and April 1997 were included. A blood sample was drawn in each case for marker assay (CA15-3 or CEA and CYFRA 21-1) on the same day as the bone scan. Two hundred and seventy-five patients were included: 118 with lung cancer and 157 with breast cancer. With regard to lung cancer, no information useful for guiding bone scan prescription was obtained through CEA and CYFRA 21-1 assays. For breast cancer, the results suggest that in asymptomatic patients, a CA15-3 level of less than 25 U/ml (upper normal value chosen as the threshold) is strongly predictive of a negative bone scan; by contrast, high tumour marker levels are predictive of neoplastic bone involvement. When a doubtful bone scan is obtained in a patient with breast cancer, a normal marker level makes it highly probable that bone scan abnormalities are not related to malignancy.