A longitudinal study of 43 families referred to a Clinical Genetic Service was undertaken to ascertain the needs and expectations of the service, from the client's perspective. Previous studies have mainly focussed on changes in knowledge or reproductive patterns as outcomes for genetic counselling. Clients were interviewed before contact with the genetic service, after the consultation, and six months later. At each interview, psychological questionnaires to assess Need for Cognitive Closure (Webster & Kruglanski, 1994), anxiety (Spielberger et aI, 1970) and the impact of the genetic condition on the family (Horowitz et aI, 1979) were used. Statistical analyses of these tools revealed that the Need for Closure is a stable entity, and that genetic counselling does not significantly influence anxiety in the client. A grounded theory approach was used to analyse the data, and the theory presented includes the client's need for certainty, the client's prior lay knowledge, and assimilation of scientific information into the family knowledge base. Clients had little prior knowledge of genetics, but had constructed lay explanations for the inheritance of the condition in their family. Scientific information was tested against the family history for validity, and where there was apparent conflict, the scientific explanation was sometimes rejected. Results of this study indicate that the majority of families are not seeking information for reproductive decision-making, but as a; means of obtaining certainty, via a diagnosis, a prognosis, or a test result. Clients defined the most important outcomes as alterations in their psychological ability to deal with the situation in their family. In this cohort, certainty was seen as helpful in enabling the client to cope, and failure to obtain certainty influenced the outcome adversely. Implications for clinical practice include the need to address the client's need for certainty, to explore the family's lay explanations as part of the genetic counselling process, and to relate explanations directly to the family experience.