A growing body of literature demonstrates the benefits of molecular pathological investigations of tumour material in the identification of individuals with hereditary non-polyposis colorectal cancer and debates the best detection strategies. This testing is novel as it is the first widespread use of somatic tissue testing to inform genetic analysis and requires the co-ordination of both histopathology and molecular genetics laboratories. However, the clinical use and experience of microsatellite instability (MSI) testing and immunohistochemical analysis have not been reported. A respondent from every cancer genetics centre in the UK (n= 24, response rate 100%) and laboratory performing MSI testing (n= 5, response rate 100%) was interviewed by telephone to ascertain test availability, testing methods, eligibility criteria and post-test management. Twenty centres (83%) offer eligible clients at least one form of tumour testing, and all use tumour testing to determine who should have access to germ line genetic testing. However, no two laboratories used the same testing methods, seven different testing strategies were applied and there was considerable variation in eligibility criteria. The implications of these variations are considered, and recommendations for the development of a consistent service for testing of somatic tissue offered.