This paper draws on Foucault to understand the changing discourse and impact of structured 'health needs assessments' on health visiting practice. Literature about this activity makes little mention of the long-standing social purposes of health visiting, which include surveillance of vulnerable and invisible populations, providing them, where needed, with help and support to access protective and supportive services. Instead, the discourse has been concerned primarily with an epidemiological focus and public health, which is associated with risk factors and assessments. The use of pre-defined needs assessment schedules suggests that health visiting activity can be sanctioned and clients' needs serviced only if they reach the threshold of pre-determined, epidemiologically-defined risk. Their effect on practice is examined through a conversation analysis of ten health visitor/client interactions using two different structured needs assessment tools. The study indicates that the health visitors, like their clients, were controlled by institutional expectations of their role; analysis of their conversations shows how they achieved the requirements of the organisational agenda. Structuring client needs and health visiting practice through the use of formal needs assessment tools emphasises the epidemiological focus of the health service above the need to arrange support for vulnerable individuals. In this respect, it serves as a marker in the continued medicalisation of health visiting.