OBJECTIVES: To assess whether healthcare professionals correctly incorporate the relevance of a favourable test outcome in a close relative when determining the level of risk for individuals at risk for Huntington's disease. DESIGN AND SETTING: Survey of clinical geneticists and genetic counsellors from 12 centres of clinical genetics (United Kingdom, 6; The Netherlands, 4; Italy, 1; Australia, 1) in May-June 2002. Participants were asked to assess risk of specific individuals in 10 pedigrees, three of which required use of Bayes' theorem. PARTICIPANTS: 71 clinical geneticists and 41 other healthcare professionals involved in genetic counselling. MAIN OUTCOME MEASURES: Proportion of respondents correctly assessing risk in the three target pedigrees; proportion of respondents who were confident of their estimate. RESULTS: 50%-64% of respondents (for the three targets separately) did not include the favourable test information and inco