Advances in genetics are allowing a greater number of clinical genetics services to be offered to individuals with family histories of particular diseases. It is important that such services are not allowed to proliferate before they have been properly evaluated in terms of both their clinical and cost-effectiveness. A randomised controlled study of a new clinical genetics service in Wales for women with a family history of breast cancer has been undertaken. The health service costs from initial consultation and counselling through to mutation testing within families were assessed, along with patient travel costs. The extra cost of specialist genetics assessment (intervention) was pound 16.36 higher per woman than assessment by a breast surgeon at a District General Hospital (control). A further cost of pound 46.42 per initially presenting woman was also generated as a result of further counselling within the families of 48, and testing within the families of 36 of the 412 original presenting women. An explanation is given here of the costing methodology as an illustration of what is involved in such an exercise.