International sharing of genomic data files arising from clinical testing of patients is essential to further improve genomic medicine. Whilst the general public are reluctant to donate DNA for research, the choices patients actually make about sharing their clinical genomic data for future re-use (research or clinical) are unknown. We ascertained the data-sharing choices of 1515 patients having genomic testing for inherited conditions or cancer treatment from clinical consent forms. To understand the experiences and preferences of these patients, surveys were administered after test consent (RR 73%). Almost all patients (98%) consented to share their data. Survey respondents' decision recall was high (90%), but poorer if English was an additional language (p < 0.001). Parents deciding on behalf of children were over-represented amongst data-sharing decliners (p = 0.047) and decliners were more likely to believe that stored data could be easily reidentified (p < 0.001). A quarter of respondents did not know if reidentification would be easy and 44% of them were concerned about this possibility. Of those willing to share data overseas (60%), 23% indicated the recipient researcher's country would affect their decision. Most respondents (89%) desired some ongoing control over research use of their data. Four preliminary data-sharing profiles emerged; their further development could inform tailored patient resources. Our results highlight considerations for establishment of systems to make clinical genomic data files available for reanalysis locally and across borders. Patients' willingness to share their data - and value of the resulting research - should encourage clinical laboratories to consider sharing data systematically for secondary uses.