We show how to use reports of cancer in family members to discover additional genetic associations or confirm previous findings in genome-wide association (GWA) studies conducted in case-control, cohort, or cross-sectional studies. Our novel family-history-based approach allows economical association studies for multiple cancers, without genotyping of relatives (as required in family studies), follow-up of participants (as required in cohort studies), or oversampling of specific cancer cases, (as required in case-control studies). We empirically evaluate the performance of the proposed family-history-based approach in studying associations with prostate and ovarian cancers, using data from GWA studies previously conducted within the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. The family-history-based method may be particularly useful for investigating genetic susceptibility to rare diseases, for which accruing cases may be very difficult, by using disease information from non-genotyped relatives of participants in multiple case-control and cohort studies designed primarily for other purposes.