Case-control studies increasingly have been used to evaluate the impact of cancer screening strategies. In this context the intent of the screening test has been to reduce cancer mortality by early detection of cancers, permitting curative therapy in some patients who would die of the disease if diagnosis were delayed until the disease was detected clinically. This phenomenon is known as secondary prevention. In an analysis of a case-control study of skin self-examination (SSE) in reducing mortality from melanoma, it was recognized that the exposure (SSE) may encourage the removal of precancerous nevi (moles), thereby reducing cancer incidence (primary prevention). This article describes an analytic strategy for obtaining separate estimates of the primary and secondary preventive impact of the screening practice. The method is focused primarily on resolving the problem of lead-time bias, caused by the artifactual advancement of the time of diagnosis in cases detected by screening.