Selection of controls with the same outward manifestations of disease as the case group has been proposed as a means of avoiding selection bias in hospital-based case-control studies. The same strategy, however, can lead to selection bias in registry-based case-control studies that use control diseases with similar manifestations whose diagnoses might have been associated with the exposure. Matching exposed and unexposed subjects by outward manifestation of disease can be used in cohort and cross-sectional studies aiming at decreasing selection bias. This strategy in these study designs may lead to overmatching, but this will not bias the relative-risk estimates. Efficiency considerations in applying this strategy require further investigation.