Using published data from screening trials, this article compares two-modality (mammography and clinical examination) and single-modality (clinical examination alone) screening by evaluating cancer detection rates, program sensitivities, mode of cancer detection in two-modality screening, nodal status at time of detection, survival 10 years post-diagnosis, and breast cancer mortality 10 years after entry. Consistently, two-modality screening achieved higher cancer detection rates and program sensitivity estimates than either modality alone; mammography alone achieved higher rates than clinical examination alone; interval cancer detection rates between screening examinations were higher following clinical examination alone than mammography alone; single-modality screening with mammography failed to detect breast cancers identified by clinical examination alone; the sensitivity of mammography was lower in younger than older women, while the reverse was true for clinical examination; and mammography identified a higher proportion of node-negative breast cancer than clinical examination. We conclude that combining clinical breast examination with mammography is desirable for women age 40-49 because mammography is less sensitive in younger than older women. Careful training and monitoring are, however, as essential with clinical examiners as with mammographers.