A one-page family history questionnaire was validated in two European areas covered by population-based cancer registries. Information on malignant tumor occurrence in first degree relatives was collected from 193 cancer patients in Trieste, Italy and from 64 in Basel, Switzerland. They were then compared with the corresponding data stored in the registries' files. The sensitivity of the questionnaire was 85% (Trieste) and 74% (Basel), the specificity was 97% in both studies and the overall accuracy 95% (Trieste) and 94% (Basel). The questionnaire is recommended for use in different geographical areas covered by population based registries for comparative analyses of cancer related family histories.